xref: /kernel/linux/linux-5.10/drivers/hv/channel_mgmt.c (revision 8c2ecf20)

// SPDX-License-Identifier: GPL-2.0-only
/*
 * Copyright (c) 2009, Microsoft Corporation.
 *
 * Authors:
 *   Haiyang Zhang <haiyangz@microsoft.com>
 *   Hank Janssen  <hjanssen@microsoft.com>
 */
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/kernel.h>
#include <linux/interrupt.h>
#include <linux/sched.h>
#include <linux/wait.h>
#include <linux/mm.h>
#include <linux/slab.h>
#include <linux/list.h>
#include <linux/module.h>
#include <linux/completion.h>
#include <linux/delay.h>
#include <linux/cpu.h>
#include <linux/hyperv.h>
#include <asm/mshyperv.h>

#include "hyperv_vmbus.h"

static void init_vp_index(struct vmbus_channel *channel);

const struct vmbus_device vmbus_devs[] = {
	/* IDE */
	{ .dev_type = HV_IDE,
	  HV_IDE_GUID,
	  .perf_device = true,
	},

	/* SCSI */
	{ .dev_type = HV_SCSI,
	  HV_SCSI_GUID,
	  .perf_device = true,
	},

	/* Fibre Channel */
	{ .dev_type = HV_FC,
	  HV_SYNTHFC_GUID,
	  .perf_device = true,
	},

	/* Synthetic NIC */
	{ .dev_type = HV_NIC,
	  HV_NIC_GUID,
	  .perf_device = true,
	},

	/* Network Direct */
	{ .dev_type = HV_ND,
	  HV_ND_GUID,
	  .perf_device = true,
	},

	/* PCIE */
	{ .dev_type = HV_PCIE,
	  HV_PCIE_GUID,
	  .perf_device = false,
	},

	/* Synthetic Frame Buffer */
	{ .dev_type = HV_FB,
	  HV_SYNTHVID_GUID,
	  .perf_device = false,
	},

	/* Synthetic Keyboard */
	{ .dev_type = HV_KBD,
	  HV_KBD_GUID,
	  .perf_device = false,
	},

	/* Synthetic MOUSE */
	{ .dev_type = HV_MOUSE,
	  HV_MOUSE_GUID,
	  .perf_device = false,
	},

	/* KVP */
	{ .dev_type = HV_KVP,
	  HV_KVP_GUID,
	  .perf_device = false,
	},

	/* Time Synch */
	{ .dev_type = HV_TS,
	  HV_TS_GUID,
	  .perf_device = false,
	},

	/* Heartbeat */
	{ .dev_type = HV_HB,
	  HV_HEART_BEAT_GUID,
	  .perf_device = false,
	},

	/* Shutdown */
	{ .dev_type = HV_SHUTDOWN,
	  HV_SHUTDOWN_GUID,
	  .perf_device = false,
	},

	/* File copy */
	{ .dev_type = HV_FCOPY,
	  HV_FCOPY_GUID,
	  .perf_device = false,
	},

	/* Backup */
	{ .dev_type = HV_BACKUP,
	  HV_VSS_GUID,
	  .perf_device = false,
	},

	/* Dynamic Memory */
	{ .dev_type = HV_DM,
	  HV_DM_GUID,
	  .perf_device = false,
	},

	/* Unknown GUID */
	{ .dev_type = HV_UNKNOWN,
	  .perf_device = false,
	},
};

static const struct {
	guid_t guid;
} vmbus_unsupported_devs[] = {
	{ HV_AVMA1_GUID },
	{ HV_AVMA2_GUID },
	{ HV_RDV_GUID	},
};

/*
 * The rescinded channel may be blocked waiting for a response from the host;
 * take care of that.
 */
static void vmbus_rescind_cleanup(struct vmbus_channel *channel)
{
	struct vmbus_channel_msginfo *msginfo;
	unsigned long flags;


	spin_lock_irqsave(&vmbus_connection.channelmsg_lock, flags);
	channel->rescind = true;
	list_for_each_entry(msginfo, &vmbus_connection.chn_msg_list,
				msglistentry) {

		if (msginfo->waiting_channel == channel) {
			complete(&msginfo->waitevent);
			break;
		}
	}
	spin_unlock_irqrestore(&vmbus_connection.channelmsg_lock, flags);
}

static bool is_unsupported_vmbus_devs(const guid_t *guid)
{
	int i;

	for (i = 0; i < ARRAY_SIZE(vmbus_unsupported_devs); i++)
		if (guid_equal(guid, &vmbus_unsupported_devs[i].guid))
			return true;
	return false;
}

static u16 hv_get_dev_type(const struct vmbus_channel *channel)
{
	const guid_t *guid = &channel->offermsg.offer.if_type;
	u16 i;

	if (is_hvsock_channel(channel) || is_unsupported_vmbus_devs(guid))
		return HV_UNKNOWN;

	for (i = HV_IDE; i < HV_UNKNOWN; i++) {
		if (guid_equal(guid, &vmbus_devs[i].guid))
			return i;
	}
	pr_info("Unknown GUID: %pUl\n", guid);
	return i;
}

/**
 * vmbus_prep_negotiate_resp() - Create default response for Negotiate message
 * @icmsghdrp: Pointer to msg header structure
 * @buf: Raw buffer channel data
 * @fw_version: The framework versions we can support.
 * @fw_vercnt: The size of @fw_version.
 * @srv_version: The service versions we can support.
 * @srv_vercnt: The size of @srv_version.
 * @nego_fw_version: The selected framework version.
 * @nego_srv_version: The selected service version.
 *
 * Note: Versions are given in decreasing order.
 *
 * Set up and fill in default negotiate response message.
 * Mainly used by Hyper-V drivers.
 */
bool vmbus_prep_negotiate_resp(struct icmsg_hdr *icmsghdrp,
				u8 *buf, const int *fw_version, int fw_vercnt,
				const int *srv_version, int srv_vercnt,
				int *nego_fw_version, int *nego_srv_version)
{
	int icframe_major, icframe_minor;
	int icmsg_major, icmsg_minor;
	int fw_major, fw_minor;
	int srv_major, srv_minor;
	int i, j;
	bool found_match = false;
	struct icmsg_negotiate *negop;

	icmsghdrp->icmsgsize = 0x10;
	negop = (struct icmsg_negotiate *)&buf[
		sizeof(struct vmbuspipe_hdr) +
		sizeof(struct icmsg_hdr)];

	icframe_major = negop->icframe_vercnt;
	icframe_minor = 0;

	icmsg_major = negop->icmsg_vercnt;
	icmsg_minor = 0;

	/*
	 * Select the framework version number we will
	 * support.
	 */

	for (i = 0; i < fw_vercnt; i++) {
		fw_major = (fw_version[i] >> 16);
		fw_minor = (fw_version[i] & 0xFFFF);

		for (j = 0; j < negop->icframe_vercnt; j++) {
			if ((negop->icversion_data[j].major == fw_major) &&
			    (negop->icversion_data[j].minor == fw_minor)) {
				icframe_major = negop->icversion_data[j].major;
				icframe_minor = negop->icversion_data[j].minor;
				found_match = true;
				break;
			}
		}

		if (found_match)
			break;
	}

	if (!found_match)
		goto fw_error;

	found_match = false;

	for (i = 0; i < srv_vercnt; i++) {
		srv_major = (srv_version[i] >> 16);
		srv_minor = (srv_version[i] & 0xFFFF);

		for (j = negop->icframe_vercnt;
			(j < negop->icframe_vercnt + negop->icmsg_vercnt);
			j++) {

			if ((negop->icversion_data[j].major == srv_major) &&
				(negop->icversion_data[j].minor == srv_minor)) {

				icmsg_major = negop->icversion_data[j].major;
				icmsg_minor = negop->icversion_data[j].minor;
				found_match = true;
				break;
			}
		}

		if (found_match)
			break;
	}

	/*
	 * Respond with the framework and service
	 * version numbers we can support.
	 */

fw_error:
	if (!found_match) {
		negop->icframe_vercnt = 0;
		negop->icmsg_vercnt = 0;
	} else {
		negop->icframe_vercnt = 1;
		negop->icmsg_vercnt = 1;
	}

	if (nego_fw_version)
		*nego_fw_version = (icframe_major << 16) | icframe_minor;

	if (nego_srv_version)
		*nego_srv_version = (icmsg_major << 16) | icmsg_minor;

	negop->icversion_data[0].major = icframe_major;
	negop->icversion_data[0].minor = icframe_minor;
	negop->icversion_data[1].major = icmsg_major;
	negop->icversion_data[1].minor = icmsg_minor;
	return found_match;
}

EXPORT_SYMBOL_GPL(vmbus_prep_negotiate_resp);

/*
 * alloc_channel - Allocate and initialize a vmbus channel object
 */
static struct vmbus_channel *alloc_channel(void)
{
	struct vmbus_channel *channel;

	channel = kzalloc(sizeof(*channel), GFP_ATOMIC);
	if (!channel)
		return NULL;

	spin_lock_init(&channel->sched_lock);
	init_completion(&channel->rescind_event);

	INIT_LIST_HEAD(&channel->sc_list);

	tasklet_init(&channel->callback_event,
		     vmbus_on_event, (unsigned long)channel);

	hv_ringbuffer_pre_init(channel);

	return channel;
}

/*
 * free_channel - Release the resources used by the vmbus channel object
 */
static void free_channel(struct vmbus_channel *channel)
{
	tasklet_kill(&channel->callback_event);
	vmbus_remove_channel_attr_group(channel);

	kobject_put(&channel->kobj);
}

void vmbus_channel_map_relid(struct vmbus_channel *channel)
{
	if (WARN_ON(channel->offermsg.child_relid >= MAX_CHANNEL_RELIDS))
		return;
	/*
	 * The mapping of the channel's relid is visible from the CPUs that
	 * execute vmbus_chan_sched() by the time that vmbus_chan_sched() will
	 * execute:
	 *
	 *  (a) In the "normal (i.e., not resuming from hibernation)" path,
	 *      the full barrier in virt_store_mb() guarantees that the store
	 *      is propagated to all CPUs before the add_channel_work work
	 *      is queued.  In turn, add_channel_work is queued before the
	 *      channel's ring buffer is allocated/initialized and the
	 *      OPENCHANNEL message for the channel is sent in vmbus_open().
	 *      Hyper-V won't start sending the interrupts for the channel
	 *      before the OPENCHANNEL message is acked.  The memory barrier
	 *      in vmbus_chan_sched() -> sync_test_and_clear_bit() ensures
	 *      that vmbus_chan_sched() must find the channel's relid in
	 *      recv_int_page before retrieving the channel pointer from the
	 *      array of channels.
	 *
	 *  (b) In the "resuming from hibernation" path, the virt_store_mb()
	 *      guarantees that the store is propagated to all CPUs before
	 *      the VMBus connection is marked as ready for the resume event
	 *      (cf. check_ready_for_resume_event()).  The interrupt handler
	 *      of the VMBus driver and vmbus_chan_sched() can not run before
	 *      vmbus_bus_resume() has completed execution (cf. resume_noirq).
	 */
	virt_store_mb(
		vmbus_connection.channels[channel->offermsg.child_relid],
		channel);
}

void vmbus_channel_unmap_relid(struct vmbus_channel *channel)
{
	if (WARN_ON(channel->offermsg.child_relid >= MAX_CHANNEL_RELIDS))
		return;
	WRITE_ONCE(
		vmbus_connection.channels[channel->offermsg.child_relid],
		NULL);
}

static void vmbus_release_relid(u32 relid)
{
	struct vmbus_channel_relid_released msg;
	int ret;

	memset(&msg, 0, sizeof(struct vmbus_channel_relid_released));
	msg.child_relid = relid;
	msg.header.msgtype = CHANNELMSG_RELID_RELEASED;
	ret = vmbus_post_msg(&msg, sizeof(struct vmbus_channel_relid_released),
			     true);

	trace_vmbus_release_relid(&msg, ret);
}

void hv_process_channel_removal(struct vmbus_channel *channel)
{
	lockdep_assert_held(&vmbus_connection.channel_mutex);
	BUG_ON(!channel->rescind);

	/*
	 * hv_process_channel_removal() could find INVALID_RELID only for
	 * hv_sock channels.  See the inline comments in vmbus_onoffer().
	 */
	WARN_ON(channel->offermsg.child_relid == INVALID_RELID &&
		!is_hvsock_channel(channel));

	/*
	 * Upon suspend, an in-use hv_sock channel is removed from the array of
	 * channels and the relid is invalidated.  After hibernation, when the
	 * user-space appplication destroys the channel, it's unnecessary and
	 * unsafe to remove the channel from the array of channels.  See also
	 * the inline comments before the call of vmbus_release_relid() below.
	 */
	if (channel->offermsg.child_relid != INVALID_RELID)
		vmbus_channel_unmap_relid(channel);

	if (channel->primary_channel == NULL)
		list_del(&channel->listentry);
	else
		list_del(&channel->sc_list);

	/*
	 * If this is a "perf" channel, updates the hv_numa_map[] masks so that
	 * init_vp_index() can (re-)use the CPU.
	 */
	if (hv_is_perf_channel(channel))
		hv_clear_alloced_cpu(channel->target_cpu);

	/*
	 * Upon suspend, an in-use hv_sock channel is marked as "rescinded" and
	 * the relid is invalidated; after hibernation, when the user-space app
	 * destroys the channel, the relid is INVALID_RELID, and in this case
	 * it's unnecessary and unsafe to release the old relid, since the same
	 * relid can refer to a completely different channel now.
	 */
	if (channel->offermsg.child_relid != INVALID_RELID)
		vmbus_release_relid(channel->offermsg.child_relid);

	free_channel(channel);
}

void vmbus_free_channels(void)
{
	struct vmbus_channel *channel, *tmp;

	list_for_each_entry_safe(channel, tmp, &vmbus_connection.chn_list,
		listentry) {
		/* hv_process_channel_removal() needs this */
		channel->rescind = true;

		vmbus_device_unregister(channel->device_obj);
	}
}

/* Note: the function can run concurrently for primary/sub channels. */
static void vmbus_add_channel_work(struct work_struct *work)
{
	struct vmbus_channel *newchannel =
		container_of(work, struct vmbus_channel, add_channel_work);
	struct vmbus_channel *primary_channel = newchannel->primary_channel;
	int ret;

	/*
	 * This state is used to indicate a successful open
	 * so that when we do close the channel normally, we
	 * can cleanup properly.
	 */
	newchannel->state = CHANNEL_OPEN_STATE;

	if (primary_channel != NULL) {
		/* newchannel is a sub-channel. */
		struct hv_device *dev = primary_channel->device_obj;

		if (vmbus_add_channel_kobj(dev, newchannel))
			goto err_deq_chan;

		if (primary_channel->sc_creation_callback != NULL)
			primary_channel->sc_creation_callback(newchannel);

		newchannel->probe_done = true;
		return;
	}

	/*
	 * Start the process of binding the primary channel to the driver
	 */
	newchannel->device_obj = vmbus_device_create(
		&newchannel->offermsg.offer.if_type,
		&newchannel->offermsg.offer.if_instance,
		newchannel);
	if (!newchannel->device_obj)
		goto err_deq_chan;

	newchannel->device_obj->device_id = newchannel->device_id;
	/*
	 * Add the new device to the bus. This will kick off device-driver
	 * binding which eventually invokes the device driver's AddDevice()
	 * method.
	 *
	 * If vmbus_device_register() fails, the 'device_obj' is freed in
	 * vmbus_device_release() as called by device_unregister() in the
	 * error path of vmbus_device_register(). In the outside error
	 * path, there's no need to free it.
	 */
	ret = vmbus_device_register(newchannel->device_obj);

	if (ret != 0) {
		pr_err("unable to add child device object (relid %d)\n",
			newchannel->offermsg.child_relid);
		goto err_deq_chan;
	}

	newchannel->probe_done = true;
	return;

err_deq_chan:
	mutex_lock(&vmbus_connection.channel_mutex);

	/*
	 * We need to set the flag, otherwise
	 * vmbus_onoffer_rescind() can be blocked.
	 */
	newchannel->probe_done = true;

	if (primary_channel == NULL)
		list_del(&newchannel->listentry);
	else
		list_del(&newchannel->sc_list);

	/* vmbus_process_offer() has mapped the channel. */
	vmbus_channel_unmap_relid(newchannel);

	mutex_unlock(&vmbus_connection.channel_mutex);

	vmbus_release_relid(newchannel->offermsg.child_relid);

	free_channel(newchannel);
}

/*
 * vmbus_process_offer - Process the offer by creating a channel/device
 * associated with this offer
 */
static void vmbus_process_offer(struct vmbus_channel *newchannel)
{
	struct vmbus_channel *channel;
	struct workqueue_struct *wq;
	bool fnew = true;

	/*
	 * Synchronize vmbus_process_offer() and CPU hotplugging:
	 *
	 * CPU1				CPU2
	 *
	 * [vmbus_process_offer()]	[Hot removal of the CPU]
	 *
	 * CPU_READ_LOCK		CPUS_WRITE_LOCK
	 * LOAD cpu_online_mask		SEARCH chn_list
	 * STORE target_cpu		LOAD target_cpu
	 * INSERT chn_list		STORE cpu_online_mask
	 * CPUS_READ_UNLOCK		CPUS_WRITE_UNLOCK
	 *
	 * Forbids: CPU1's LOAD from *not* seing CPU2's STORE &&
	 * 		CPU2's SEARCH from *not* seeing CPU1's INSERT
	 *
	 * Forbids: CPU2's SEARCH from seeing CPU1's INSERT &&
	 * 		CPU2's LOAD from *not* seing CPU1's STORE
	 */
	cpus_read_lock();

	/*
	 * Serializes the modifications of the chn_list list as well as
	 * the accesses to next_numa_node_id in init_vp_index().
	 */
	mutex_lock(&vmbus_connection.channel_mutex);

	init_vp_index(newchannel);

	/* Remember the channels that should be cleaned up upon suspend. */
	if (is_hvsock_channel(newchannel) || is_sub_channel(newchannel))
		atomic_inc(&vmbus_connection.nr_chan_close_on_suspend);

	/*
	 * Now that we have acquired the channel_mutex,
	 * we can release the potentially racing rescind thread.
	 */
	atomic_dec(&vmbus_connection.offer_in_progress);

	list_for_each_entry(channel, &vmbus_connection.chn_list, listentry) {
		if (guid_equal(&channel->offermsg.offer.if_type,
			       &newchannel->offermsg.offer.if_type) &&
		    guid_equal(&channel->offermsg.offer.if_instance,
			       &newchannel->offermsg.offer.if_instance)) {
			fnew = false;
			break;
		}
	}

	if (fnew) {
		list_add_tail(&newchannel->listentry,
			      &vmbus_connection.chn_list);
	} else {
		/*
		 * Check to see if this is a valid sub-channel.
		 */
		if (newchannel->offermsg.offer.sub_channel_index == 0) {
			mutex_unlock(&vmbus_connection.channel_mutex);
			cpus_read_unlock();
			/*
			 * Don't call free_channel(), because newchannel->kobj
			 * is not initialized yet.
			 */
			kfree(newchannel);
			WARN_ON_ONCE(1);
			return;
		}
		/*
		 * Process the sub-channel.
		 */
		newchannel->primary_channel = channel;
		list_add_tail(&newchannel->sc_list, &channel->sc_list);
	}

	vmbus_channel_map_relid(newchannel);

	mutex_unlock(&vmbus_connection.channel_mutex);
	cpus_read_unlock();

	/*
	 * vmbus_process_offer() mustn't call channel->sc_creation_callback()
	 * directly for sub-channels, because sc_creation_callback() ->
	 * vmbus_open() may never get the host's response to the
	 * OPEN_CHANNEL message (the host may rescind a channel at any time,
	 * e.g. in the case of hot removing a NIC), and vmbus_onoffer_rescind()
	 * may not wake up the vmbus_open() as it's blocked due to a non-zero
	 * vmbus_connection.offer_in_progress, and finally we have a deadlock.
	 *
	 * The above is also true for primary channels, if the related device
	 * drivers use sync probing mode by default.
	 *
	 * And, usually the handling of primary channels and sub-channels can
	 * depend on each other, so we should offload them to different
	 * workqueues to avoid possible deadlock, e.g. in sync-probing mode,
	 * NIC1's netvsc_subchan_work() can race with NIC2's netvsc_probe() ->
	 * rtnl_lock(), and causes deadlock: the former gets the rtnl_lock
	 * and waits for all the sub-channels to appear, but the latter
	 * can't get the rtnl_lock and this blocks the handling of
	 * sub-channels.
	 */
	INIT_WORK(&newchannel->add_channel_work, vmbus_add_channel_work);
	wq = fnew ? vmbus_connection.handle_primary_chan_wq :
		    vmbus_connection.handle_sub_chan_wq;
	queue_work(wq, &newchannel->add_channel_work);
}

/*
 * We use this state to statically distribute the channel interrupt load.
 */
static int next_numa_node_id;

/*
 * Starting with Win8, we can statically distribute the incoming
 * channel interrupt load by binding a channel to VCPU.
 *
 * For pre-win8 hosts or non-performance critical channels we assign the
 * VMBUS_CONNECT_CPU.
 *
 * Starting with win8, performance critical channels will be distributed
 * evenly among all the available NUMA nodes.  Once the node is assigned,
 * we will assign the CPU based on a simple round robin scheme.
 */
static void init_vp_index(struct vmbus_channel *channel)
{
	bool perf_chn = hv_is_perf_channel(channel);
	cpumask_var_t available_mask;
	struct cpumask *alloced_mask;
	u32 target_cpu;
	int numa_node;

	if ((vmbus_proto_version == VERSION_WS2008) ||
	    (vmbus_proto_version == VERSION_WIN7) || (!perf_chn) ||
	    !alloc_cpumask_var(&available_mask, GFP_KERNEL)) {
		/*
		 * Prior to win8, all channel interrupts are
		 * delivered on VMBUS_CONNECT_CPU.
		 * Also if the channel is not a performance critical
		 * channel, bind it to VMBUS_CONNECT_CPU.
		 * In case alloc_cpumask_var() fails, bind it to
		 * VMBUS_CONNECT_CPU.
		 */
		channel->target_cpu = VMBUS_CONNECT_CPU;
		if (perf_chn)
			hv_set_alloced_cpu(VMBUS_CONNECT_CPU);
		return;
	}

	while (true) {
		numa_node = next_numa_node_id++;
		if (numa_node == nr_node_ids) {
			next_numa_node_id = 0;
			continue;
		}
		if (cpumask_empty(cpumask_of_node(numa_node)))
			continue;
		break;
	}
	alloced_mask = &hv_context.hv_numa_map[numa_node];

	if (cpumask_weight(alloced_mask) ==
	    cpumask_weight(cpumask_of_node(numa_node))) {
		/*
		 * We have cycled through all the CPUs in the node;
		 * reset the alloced map.
		 */
		cpumask_clear(alloced_mask);
	}

	cpumask_xor(available_mask, alloced_mask, cpumask_of_node(numa_node));

	target_cpu = cpumask_first(available_mask);
	cpumask_set_cpu(target_cpu, alloced_mask);

	channel->target_cpu = target_cpu;

	free_cpumask_var(available_mask);
}

#define UNLOAD_DELAY_UNIT_MS	10		/* 10 milliseconds */
#define UNLOAD_WAIT_MS		(100*1000)	/* 100 seconds */
#define UNLOAD_WAIT_LOOPS	(UNLOAD_WAIT_MS/UNLOAD_DELAY_UNIT_MS)
#define UNLOAD_MSG_MS		(5*1000)	/* Every 5 seconds */
#define UNLOAD_MSG_LOOPS	(UNLOAD_MSG_MS/UNLOAD_DELAY_UNIT_MS)

static void vmbus_wait_for_unload(void)
{
	int cpu;
	void *page_addr;
	struct hv_message *msg;
	struct vmbus_channel_message_header *hdr;
	u32 message_type, i;

	/*
	 * CHANNELMSG_UNLOAD_RESPONSE is always delivered to the CPU which was
	 * used for initial contact or to CPU0 depending on host version. When
	 * we're crashing on a different CPU let's hope that IRQ handler on
	 * the cpu which receives CHANNELMSG_UNLOAD_RESPONSE is still
	 * functional and vmbus_unload_response() will complete
	 * vmbus_connection.unload_event. If not, the last thing we can do is
	 * read message pages for all CPUs directly.
	 *
	 * Wait up to 100 seconds since an Azure host must writeback any dirty
	 * data in its disk cache before the VMbus UNLOAD request will
	 * complete. This flushing has been empirically observed to take up
	 * to 50 seconds in cases with a lot of dirty data, so allow additional
	 * leeway and for inaccuracies in mdelay(). But eventually time out so
	 * that the panic path can't get hung forever in case the response
	 * message isn't seen.
	 */
	for (i = 1; i <= UNLOAD_WAIT_LOOPS; i++) {
		if (completion_done(&vmbus_connection.unload_event))
			goto completed;

		for_each_present_cpu(cpu) {
			struct hv_per_cpu_context *hv_cpu
				= per_cpu_ptr(hv_context.cpu_context, cpu);

			/*
			 * In a CoCo VM the synic_message_page is not allocated
			 * in hv_synic_alloc(). Instead it is set/cleared in
			 * hv_synic_enable_regs() and hv_synic_disable_regs()
			 * such that it is set only when the CPU is online. If
			 * not all present CPUs are online, the message page
			 * might be NULL, so skip such CPUs.
			 */
			page_addr = hv_cpu->synic_message_page;
			if (!page_addr)
				continue;

			msg = (struct hv_message *)page_addr
				+ VMBUS_MESSAGE_SINT;

			message_type = READ_ONCE(msg->header.message_type);
			if (message_type == HVMSG_NONE)
				continue;

			hdr = (struct vmbus_channel_message_header *)
				msg->u.payload;

			if (hdr->msgtype == CHANNELMSG_UNLOAD_RESPONSE)
				complete(&vmbus_connection.unload_event);

			vmbus_signal_eom(msg, message_type);
		}

		/*
		 * Give a notice periodically so someone watching the
		 * serial output won't think it is completely hung.
		 */
		if (!(i % UNLOAD_MSG_LOOPS))
			pr_notice("Waiting for VMBus UNLOAD to complete\n");

		mdelay(UNLOAD_DELAY_UNIT_MS);
	}
	pr_err("Continuing even though VMBus UNLOAD did not complete\n");

completed:
	/*
	 * We're crashing and already got the UNLOAD_RESPONSE, cleanup all
	 * maybe-pending messages on all CPUs to be able to receive new
	 * messages after we reconnect.
	 */
	for_each_present_cpu(cpu) {
		struct hv_per_cpu_context *hv_cpu
			= per_cpu_ptr(hv_context.cpu_context, cpu);

		page_addr = hv_cpu->synic_message_page;
		if (!page_addr)
			continue;

		msg = (struct hv_message *)page_addr + VMBUS_MESSAGE_SINT;
		msg->header.message_type = HVMSG_NONE;
	}
}

/*
 * vmbus_unload_response - Handler for the unload response.
 */
static void vmbus_unload_response(struct vmbus_channel_message_header *hdr)
{
	/*
	 * This is a global event; just wakeup the waiting thread.
	 * Once we successfully unload, we can cleanup the monitor state.
	 */
	complete(&vmbus_connection.unload_event);
}

void vmbus_initiate_unload(bool crash)
{
	struct vmbus_channel_message_header hdr;

	if (xchg(&vmbus_connection.conn_state, DISCONNECTED) == DISCONNECTED)
		return;

	/* Pre-Win2012R2 hosts don't support reconnect */
	if (vmbus_proto_version < VERSION_WIN8_1)
		return;

	init_completion(&vmbus_connection.unload_event);
	memset(&hdr, 0, sizeof(struct vmbus_channel_message_header));
	hdr.msgtype = CHANNELMSG_UNLOAD;
	vmbus_post_msg(&hdr, sizeof(struct vmbus_channel_message_header),
		       !crash);

	/*
	 * vmbus_initiate_unload() is also called on crash and the crash can be
	 * happening in an interrupt context, where scheduling is impossible.
	 */
	if (!crash)
		wait_for_completion(&vmbus_connection.unload_event);
	else
		vmbus_wait_for_unload();
}

static void check_ready_for_resume_event(void)
{
	/*
	 * If all the old primary channels have been fixed up, then it's safe
	 * to resume.
	 */
	if (atomic_dec_and_test(&vmbus_connection.nr_chan_fixup_on_resume))
		complete(&vmbus_connection.ready_for_resume_event);
}

static void vmbus_setup_channel_state(struct vmbus_channel *channel,
				      struct vmbus_channel_offer_channel *offer)
{
	/*
	 * Setup state for signalling the host.
	 */
	channel->sig_event = VMBUS_EVENT_CONNECTION_ID;

	if (vmbus_proto_version != VERSION_WS2008) {
		channel->is_dedicated_interrupt =
				(offer->is_dedicated_interrupt != 0);
		channel->sig_event = offer->connection_id;
	}

	memcpy(&channel->offermsg, offer,
	       sizeof(struct vmbus_channel_offer_channel));
	channel->monitor_grp = (u8)offer->monitorid / 32;
	channel->monitor_bit = (u8)offer->monitorid % 32;
	channel->device_id = hv_get_dev_type(channel);
}

/*
 * find_primary_channel_by_offer - Get the channel object given the new offer.
 * This is only used in the resume path of hibernation.
 */
static struct vmbus_channel *
find_primary_channel_by_offer(const struct vmbus_channel_offer_channel *offer)
{
	struct vmbus_channel *channel = NULL, *iter;
	const guid_t *inst1, *inst2;

	/* Ignore sub-channel offers. */
	if (offer->offer.sub_channel_index != 0)
		return NULL;

	mutex_lock(&vmbus_connection.channel_mutex);

	list_for_each_entry(iter, &vmbus_connection.chn_list, listentry) {
		inst1 = &iter->offermsg.offer.if_instance;
		inst2 = &offer->offer.if_instance;

		if (guid_equal(inst1, inst2)) {
			channel = iter;
			break;
		}
	}

	mutex_unlock(&vmbus_connection.channel_mutex);

	return channel;
}

/*
 * vmbus_onoffer - Handler for channel offers from vmbus in parent partition.
 *
 */
static void vmbus_onoffer(struct vmbus_channel_message_header *hdr)
{
	struct vmbus_channel_offer_channel *offer;
	struct vmbus_channel *oldchannel, *newchannel;
	size_t offer_sz;

	offer = (struct vmbus_channel_offer_channel *)hdr;

	trace_vmbus_onoffer(offer);

	oldchannel = find_primary_channel_by_offer(offer);

	if (oldchannel != NULL) {
		/*
		 * We're resuming from hibernation: all the sub-channel and
		 * hv_sock channels we had before the hibernation should have
		 * been cleaned up, and now we must be seeing a re-offered
		 * primary channel that we had before the hibernation.
		 */

		/*
		 * { Initially: channel relid = INVALID_RELID,
		 *		channels[valid_relid] = NULL }
		 *
		 * CPU1					CPU2
		 *
		 * [vmbus_onoffer()]			[vmbus_device_release()]
		 *
		 * LOCK channel_mutex			LOCK channel_mutex
		 * STORE channel relid = valid_relid	LOAD r1 = channel relid
		 * MAP_RELID channel			if (r1 != INVALID_RELID)
		 * UNLOCK channel_mutex			  UNMAP_RELID channel
		 *					UNLOCK channel_mutex
		 *
		 * Forbids: r1 == valid_relid &&
		 * 		channels[valid_relid] == channel
		 *
		 * Note.  r1 can be INVALID_RELID only for an hv_sock channel.
		 * None of the hv_sock channels which were present before the
		 * suspend are re-offered upon the resume.  See the WARN_ON()
		 * in hv_process_channel_removal().
		 */
		mutex_lock(&vmbus_connection.channel_mutex);

		atomic_dec(&vmbus_connection.offer_in_progress);

		WARN_ON(oldchannel->offermsg.child_relid != INVALID_RELID);
		/* Fix up the relid. */
		oldchannel->offermsg.child_relid = offer->child_relid;

		offer_sz = sizeof(*offer);
		if (memcmp(offer, &oldchannel->offermsg, offer_sz) != 0) {
			/*
			 * This is not an error, since the host can also change
			 * the other field(s) of the offer, e.g. on WS RS5
			 * (Build 17763), the offer->connection_id of the
			 * Mellanox VF vmbus device can change when the host
			 * reoffers the device upon resume.
			 */
			pr_debug("vmbus offer changed: relid=%d\n",
				 offer->child_relid);

			print_hex_dump_debug("Old vmbus offer: ",
					     DUMP_PREFIX_OFFSET, 16, 4,
					     &oldchannel->offermsg, offer_sz,
					     false);
			print_hex_dump_debug("New vmbus offer: ",
					     DUMP_PREFIX_OFFSET, 16, 4,
					     offer, offer_sz, false);

			/* Fix up the old channel. */
			vmbus_setup_channel_state(oldchannel, offer);
		}

		/* Add the channel back to the array of channels. */
		vmbus_channel_map_relid(oldchannel);
		check_ready_for_resume_event();

		mutex_unlock(&vmbus_connection.channel_mutex);
		return;
	}

	/* Allocate the channel object and save this offer. */
	newchannel = alloc_channel();
	if (!newchannel) {
		vmbus_release_relid(offer->child_relid);
		atomic_dec(&vmbus_connection.offer_in_progress);
		pr_err("Unable to allocate channel object\n");
		return;
	}

	vmbus_setup_channel_state(newchannel, offer);

	vmbus_process_offer(newchannel);
}

static void check_ready_for_suspend_event(void)
{
	/*
	 * If all the sub-channels or hv_sock channels have been cleaned up,
	 * then it's safe to suspend.
	 */
	if (atomic_dec_and_test(&vmbus_connection.nr_chan_close_on_suspend))
		complete(&vmbus_connection.ready_for_suspend_event);
}

/*
 * vmbus_onoffer_rescind - Rescind offer handler.
 *
 * We queue a work item to process this offer synchronously
 */
static void vmbus_onoffer_rescind(struct vmbus_channel_message_header *hdr)
{
	struct vmbus_channel_rescind_offer *rescind;
	struct vmbus_channel *channel;
	struct device *dev;
	bool clean_up_chan_for_suspend;

	rescind = (struct vmbus_channel_rescind_offer *)hdr;

	trace_vmbus_onoffer_rescind(rescind);

	/*
	 * The offer msg and the corresponding rescind msg
	 * from the host are guranteed to be ordered -
	 * offer comes in first and then the rescind.
	 * Since we process these events in work elements,
	 * and with preemption, we may end up processing
	 * the events out of order.  We rely on the synchronization
	 * provided by offer_in_progress and by channel_mutex for
	 * ordering these events:
	 *
	 * { Initially: offer_in_progress = 1 }
	 *
	 * CPU1				CPU2
	 *
	 * [vmbus_onoffer()]		[vmbus_onoffer_rescind()]
	 *
	 * LOCK channel_mutex		WAIT_ON offer_in_progress == 0
	 * DECREMENT offer_in_progress	LOCK channel_mutex
	 * STORE channels[]		LOAD channels[]
	 * UNLOCK channel_mutex		UNLOCK channel_mutex
	 *
	 * Forbids: CPU2's LOAD from *not* seeing CPU1's STORE
	 */

	while (atomic_read(&vmbus_connection.offer_in_progress) != 0) {
		/*
		 * We wait here until any channel offer is currently
		 * being processed.
		 */
		msleep(1);
	}

	mutex_lock(&vmbus_connection.channel_mutex);
	channel = relid2channel(rescind->child_relid);
	mutex_unlock(&vmbus_connection.channel_mutex);

	if (channel == NULL) {
		/*
		 * We failed in processing the offer message;
		 * we would have cleaned up the relid in that
		 * failure path.
		 */
		return;
	}

	clean_up_chan_for_suspend = is_hvsock_channel(channel) ||
				    is_sub_channel(channel);
	/*
	 * Before setting channel->rescind in vmbus_rescind_cleanup(), we
	 * should make sure the channel callback is not running any more.
	 */
	vmbus_reset_channel_cb(channel);

	/*
	 * Now wait for offer handling to complete.
	 */
	vmbus_rescind_cleanup(channel);
	while (READ_ONCE(channel->probe_done) == false) {
		/*
		 * We wait here until any channel offer is currently
		 * being processed.
		 */
		msleep(1);
	}

	/*
	 * At this point, the rescind handling can proceed safely.
	 */

	if (channel->device_obj) {
		if (channel->chn_rescind_callback) {
			channel->chn_rescind_callback(channel);

			if (clean_up_chan_for_suspend)
				check_ready_for_suspend_event();

			return;
		}
		/*
		 * We will have to unregister this device from the
		 * driver core.
		 */
		dev = get_device(&channel->device_obj->device);
		if (dev) {
			vmbus_device_unregister(channel->device_obj);
			put_device(dev);
		}
	} else if (channel->primary_channel != NULL) {
		/*
		 * Sub-channel is being rescinded. Following is the channel
		 * close sequence when initiated from the driveri (refer to
		 * vmbus_close() for details):
		 * 1. Close all sub-channels first
		 * 2. Then close the primary channel.
		 */
		mutex_lock(&vmbus_connection.channel_mutex);
		if (channel->state == CHANNEL_OPEN_STATE) {
			/*
			 * The channel is currently not open;
			 * it is safe for us to cleanup the channel.
			 */
			hv_process_channel_removal(channel);
		} else {
			complete(&channel->rescind_event);
		}
		mutex_unlock(&vmbus_connection.channel_mutex);
	}

	/* The "channel" may have been freed. Do not access it any longer. */

	if (clean_up_chan_for_suspend)
		check_ready_for_suspend_event();
}

void vmbus_hvsock_device_unregister(struct vmbus_channel *channel)
{
	BUG_ON(!is_hvsock_channel(channel));

	/* We always get a rescind msg when a connection is closed. */
	while (!READ_ONCE(channel->probe_done) || !READ_ONCE(channel->rescind))
		msleep(1);

	vmbus_device_unregister(channel->device_obj);
}
EXPORT_SYMBOL_GPL(vmbus_hvsock_device_unregister);


/*
 * vmbus_onoffers_delivered -
 * This is invoked when all offers have been delivered.
 *
 * Nothing to do here.
 */
static void vmbus_onoffers_delivered(
			struct vmbus_channel_message_header *hdr)
{
}

/*
 * vmbus_onopen_result - Open result handler.
 *
 * This is invoked when we received a response to our channel open request.
 * Find the matching request, copy the response and signal the requesting
 * thread.
 */
static void vmbus_onopen_result(struct vmbus_channel_message_header *hdr)
{
	struct vmbus_channel_open_result *result;
	struct vmbus_channel_msginfo *msginfo;
	struct vmbus_channel_message_header *requestheader;
	struct vmbus_channel_open_channel *openmsg;
	unsigned long flags;

	result = (struct vmbus_channel_open_result *)hdr;

	trace_vmbus_onopen_result(result);

	/*
	 * Find the open msg, copy the result and signal/unblock the wait event
	 */
	spin_lock_irqsave(&vmbus_connection.channelmsg_lock, flags);

	list_for_each_entry(msginfo, &vmbus_connection.chn_msg_list,
				msglistentry) {
		requestheader =
			(struct vmbus_channel_message_header *)msginfo->msg;

		if (requestheader->msgtype == CHANNELMSG_OPENCHANNEL) {
			openmsg =
			(struct vmbus_channel_open_channel *)msginfo->msg;
			if (openmsg->child_relid == result->child_relid &&
			    openmsg->openid == result->openid) {
				memcpy(&msginfo->response.open_result,
				       result,
				       sizeof(
					struct vmbus_channel_open_result));
				complete(&msginfo->waitevent);
				break;
			}
		}
	}
	spin_unlock_irqrestore(&vmbus_connection.channelmsg_lock, flags);
}

/*
 * vmbus_ongpadl_created - GPADL created handler.
 *
 * This is invoked when we received a response to our gpadl create request.
 * Find the matching request, copy the response and signal the requesting
 * thread.
 */
static void vmbus_ongpadl_created(struct vmbus_channel_message_header *hdr)
{
	struct vmbus_channel_gpadl_created *gpadlcreated;
	struct vmbus_channel_msginfo *msginfo;
	struct vmbus_channel_message_header *requestheader;
	struct vmbus_channel_gpadl_header *gpadlheader;
	unsigned long flags;

	gpadlcreated = (struct vmbus_channel_gpadl_created *)hdr;

	trace_vmbus_ongpadl_created(gpadlcreated);

	/*
	 * Find the establish msg, copy the result and signal/unblock the wait
	 * event
	 */
	spin_lock_irqsave(&vmbus_connection.channelmsg_lock, flags);

	list_for_each_entry(msginfo, &vmbus_connection.chn_msg_list,
				msglistentry) {
		requestheader =
			(struct vmbus_channel_message_header *)msginfo->msg;

		if (requestheader->msgtype == CHANNELMSG_GPADL_HEADER) {
			gpadlheader =
			(struct vmbus_channel_gpadl_header *)requestheader;

			if ((gpadlcreated->child_relid ==
			     gpadlheader->child_relid) &&
			    (gpadlcreated->gpadl == gpadlheader->gpadl)) {
				memcpy(&msginfo->response.gpadl_created,
				       gpadlcreated,
				       sizeof(
					struct vmbus_channel_gpadl_created));
				complete(&msginfo->waitevent);
				break;
			}
		}
	}
	spin_unlock_irqrestore(&vmbus_connection.channelmsg_lock, flags);
}

/*
 * vmbus_ongpadl_torndown - GPADL torndown handler.
 *
 * This is invoked when we received a response to our gpadl teardown request.
 * Find the matching request, copy the response and signal the requesting
 * thread.
 */
static void vmbus_ongpadl_torndown(
			struct vmbus_channel_message_header *hdr)
{
	struct vmbus_channel_gpadl_torndown *gpadl_torndown;
	struct vmbus_channel_msginfo *msginfo;
	struct vmbus_channel_message_header *requestheader;
	struct vmbus_channel_gpadl_teardown *gpadl_teardown;
	unsigned long flags;

	gpadl_torndown = (struct vmbus_channel_gpadl_torndown *)hdr;

	trace_vmbus_ongpadl_torndown(gpadl_torndown);

	/*
	 * Find the open msg, copy the result and signal/unblock the wait event
	 */
	spin_lock_irqsave(&vmbus_connection.channelmsg_lock, flags);

	list_for_each_entry(msginfo, &vmbus_connection.chn_msg_list,
				msglistentry) {
		requestheader =
			(struct vmbus_channel_message_header *)msginfo->msg;

		if (requestheader->msgtype == CHANNELMSG_GPADL_TEARDOWN) {
			gpadl_teardown =
			(struct vmbus_channel_gpadl_teardown *)requestheader;

			if (gpadl_torndown->gpadl == gpadl_teardown->gpadl) {
				memcpy(&msginfo->response.gpadl_torndown,
				       gpadl_torndown,
				       sizeof(
					struct vmbus_channel_gpadl_torndown));
				complete(&msginfo->waitevent);
				break;
			}
		}
	}
	spin_unlock_irqrestore(&vmbus_connection.channelmsg_lock, flags);
}

/*
 * vmbus_onversion_response - Version response handler
 *
 * This is invoked when we received a response to our initiate contact request.
 * Find the matching request, copy the response and signal the requesting
 * thread.
 */
static void vmbus_onversion_response(
		struct vmbus_channel_message_header *hdr)
{
	struct vmbus_channel_msginfo *msginfo;
	struct vmbus_channel_message_header *requestheader;
	struct vmbus_channel_version_response *version_response;
	unsigned long flags;

	version_response = (struct vmbus_channel_version_response *)hdr;

	trace_vmbus_onversion_response(version_response);

	spin_lock_irqsave(&vmbus_connection.channelmsg_lock, flags);

	list_for_each_entry(msginfo, &vmbus_connection.chn_msg_list,
				msglistentry) {
		requestheader =
			(struct vmbus_channel_message_header *)msginfo->msg;

		if (requestheader->msgtype ==
		    CHANNELMSG_INITIATE_CONTACT) {
			memcpy(&msginfo->response.version_response,
			      version_response,
			      sizeof(struct vmbus_channel_version_response));
			complete(&msginfo->waitevent);
		}
	}
	spin_unlock_irqrestore(&vmbus_connection.channelmsg_lock, flags);
}

/* Channel message dispatch table */
const struct vmbus_channel_message_table_entry
channel_message_table[CHANNELMSG_COUNT] = {
	{ CHANNELMSG_INVALID,			0, NULL, 0},
	{ CHANNELMSG_OFFERCHANNEL,		0, vmbus_onoffer,
		sizeof(struct vmbus_channel_offer_channel)},
	{ CHANNELMSG_RESCIND_CHANNELOFFER,	0, vmbus_onoffer_rescind,
		sizeof(struct vmbus_channel_rescind_offer) },
	{ CHANNELMSG_REQUESTOFFERS,		0, NULL, 0},
	{ CHANNELMSG_ALLOFFERS_DELIVERED,	1, vmbus_onoffers_delivered, 0},
	{ CHANNELMSG_OPENCHANNEL,		0, NULL, 0},
	{ CHANNELMSG_OPENCHANNEL_RESULT,	1, vmbus_onopen_result,
		sizeof(struct vmbus_channel_open_result)},
	{ CHANNELMSG_CLOSECHANNEL,		0, NULL, 0},
	{ CHANNELMSG_GPADL_HEADER,		0, NULL, 0},
	{ CHANNELMSG_GPADL_BODY,		0, NULL, 0},
	{ CHANNELMSG_GPADL_CREATED,		1, vmbus_ongpadl_created,
		sizeof(struct vmbus_channel_gpadl_created)},
	{ CHANNELMSG_GPADL_TEARDOWN,		0, NULL, 0},
	{ CHANNELMSG_GPADL_TORNDOWN,		1, vmbus_ongpadl_torndown,
		sizeof(struct vmbus_channel_gpadl_torndown) },
	{ CHANNELMSG_RELID_RELEASED,		0, NULL, 0},
	{ CHANNELMSG_INITIATE_CONTACT,		0, NULL, 0},
	{ CHANNELMSG_VERSION_RESPONSE,		1, vmbus_onversion_response,
		sizeof(struct vmbus_channel_version_response)},
	{ CHANNELMSG_UNLOAD,			0, NULL, 0},
	{ CHANNELMSG_UNLOAD_RESPONSE,		1, vmbus_unload_response, 0},
	{ CHANNELMSG_18,			0, NULL, 0},
	{ CHANNELMSG_19,			0, NULL, 0},
	{ CHANNELMSG_20,			0, NULL, 0},
	{ CHANNELMSG_TL_CONNECT_REQUEST,	0, NULL, 0},
	{ CHANNELMSG_MODIFYCHANNEL,		0, NULL, 0},
	{ CHANNELMSG_TL_CONNECT_RESULT,		0, NULL, 0},
};

/*
 * vmbus_onmessage - Handler for channel protocol messages.
 *
 * This is invoked in the vmbus worker thread context.
 */
void vmbus_onmessage(struct vmbus_channel_message_header *hdr)
{
	trace_vmbus_on_message(hdr);

	/*
	 * vmbus_on_msg_dpc() makes sure the hdr->msgtype here can not go
	 * out of bound and the message_handler pointer can not be NULL.
	 */
	channel_message_table[hdr->msgtype].message_handler(hdr);
}

/*
 * vmbus_request_offers - Send a request to get all our pending offers.
 */
int vmbus_request_offers(void)
{
	struct vmbus_channel_message_header *msg;
	struct vmbus_channel_msginfo *msginfo;
	int ret;

	msginfo = kmalloc(sizeof(*msginfo) +
			  sizeof(struct vmbus_channel_message_header),
			  GFP_KERNEL);
	if (!msginfo)
		return -ENOMEM;

	msg = (struct vmbus_channel_message_header *)msginfo->msg;

	msg->msgtype = CHANNELMSG_REQUESTOFFERS;

	ret = vmbus_post_msg(msg, sizeof(struct vmbus_channel_message_header),
			     true);

	trace_vmbus_request_offers(ret);

	if (ret != 0) {
		pr_err("Unable to request offers - %d\n", ret);

		goto cleanup;
	}

cleanup:
	kfree(msginfo);

	return ret;
}

static void invoke_sc_cb(struct vmbus_channel *primary_channel)
{
	struct list_head *cur, *tmp;
	struct vmbus_channel *cur_channel;

	if (primary_channel->sc_creation_callback == NULL)
		return;

	list_for_each_safe(cur, tmp, &primary_channel->sc_list) {
		cur_channel = list_entry(cur, struct vmbus_channel, sc_list);

		primary_channel->sc_creation_callback(cur_channel);
	}
}

void vmbus_set_sc_create_callback(struct vmbus_channel *primary_channel,
				void (*sc_cr_cb)(struct vmbus_channel *new_sc))
{
	primary_channel->sc_creation_callback = sc_cr_cb;
}
EXPORT_SYMBOL_GPL(vmbus_set_sc_create_callback);

bool vmbus_are_subchannels_present(struct vmbus_channel *primary)
{
	bool ret;

	ret = !list_empty(&primary->sc_list);

	if (ret) {
		/*
		 * Invoke the callback on sub-channel creation.
		 * This will present a uniform interface to the
		 * clients.
		 */
		invoke_sc_cb(primary);
	}

	return ret;
}
EXPORT_SYMBOL_GPL(vmbus_are_subchannels_present);

void vmbus_set_chn_rescind_callback(struct vmbus_channel *channel,
		void (*chn_rescind_cb)(struct vmbus_channel *))
{
	channel->chn_rescind_callback = chn_rescind_cb;
}
EXPORT_SYMBOL_GPL(vmbus_set_chn_rescind_callback);