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Date:	Sun, 10 Aug 2014 21:45:59 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Razya Ladelsky <razya@...ibm.com>
Cc:	kvm@...r.kernel.org, GLIKSON@...ibm.com, ERANRA@...ibm.com,
	YOSSIKU@...ibm.com, JOELN@...ibm.com, abel.gordon@...il.com,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] vhost: Add polling mode

On Sun, Aug 10, 2014 at 11:30:35AM +0300, Razya Ladelsky wrote:
> From: Razya Ladelsky <razya@...ibm.com>
> Date: Thu, 31 Jul 2014 09:47:20 +0300
> Subject: [PATCH] vhost: Add polling mode
> 
> When vhost is waiting for buffers from the guest driver (e.g., more packets to
> send in vhost-net's transmit queue), it normally goes to sleep and waits for the
> guest to "kick" it. This kick involves a PIO in the guest, and therefore an exit
> (and possibly userspace involvement in translating this PIO exit into a file
> descriptor event), all of which hurts performance.
> 
> If the system is under-utilized (has cpu time to spare), vhost can continuously
> poll the virtqueues for new buffers, and avoid asking the guest to kick us.
> This patch adds an optional polling mode to vhost, that can be enabled via a
> kernel module parameter, "poll_start_rate".
> 
> When polling is active for a virtqueue, the guest is asked to disable
> notification (kicks), and the worker thread continuously checks for new buffers.
> When it does discover new buffers, it simulates a "kick" by invoking the
> underlying backend driver (such as vhost-net), which thinks it got a real kick
> from the guest, and acts accordingly. If the underlying driver asks not to be
> kicked, we disable polling on this virtqueue.
> 
> We start polling on a virtqueue when we notice it has work to do. Polling on
> this virtqueue is later disabled after 3 seconds of polling turning up no new
> work, as in this case we are better off returning to the exit-based notification
> mechanism. The default timeout of 3 seconds can be changed with the
> "poll_stop_idle" kernel module parameter.
> 
> This polling approach makes lot of sense for new HW with posted-interrupts for
> which we have exitless host-to-guest notifications. But even with support for
> posted interrupts, guest-to-host communication still causes exits. Polling adds
> the missing part.
> 
> When systems are overloaded, there won't be enough cpu time for the various
> vhost threads to poll their guests' devices. For these scenarios, we plan to add
> support for vhost threads that can be shared by multiple devices, even of
> multiple vms.
> Our ultimate goal is to implement the I/O acceleration features described in:
> KVM Forum 2013: Efficient and Scalable Virtio (by Abel Gordon)
> https://www.youtube.com/watch?v=9EyweibHfEs
> and
> https://www.mail-archive.com/kvm@vger.kernel.org/msg98179.html
> 
> I ran some experiments with TCP stream netperf and filebench (having 2 threads
> performing random reads) benchmarks on an IBM System x3650 M4.
> I have two machines, A and B. A hosts the vms, B runs the netserver.
> The vms (on A) run netperf, its destination server is running on B.
> All runs loaded the guests in a way that they were (cpu) saturated. For example,
> I ran netperf with 64B messages, which is heavily loading the vm (which is why
> its throughput is low).
> The idea was to get it 100% loaded, so we can see that the polling is getting it
> to produce higher throughput.

And, did your tests actually produce 100% load on both host CPUs?

> The system had two cores per guest, as to allow for both the vcpu and the vhost
> thread to run concurrently for maximum throughput (but I didn't pin the threads
> to specific cores).
> My experiments were fair in a sense that for both cases, with or without
> polling, I run both threads, vcpu and vhost, on 2 cores (set their affinity that
> way). The only difference was whether polling was enabled/disabled.
> 
> Results:
> 
> Netperf, 1 vm:
> The polling patch improved throughput by ~33% (1516 MB/sec -> 2046 MB/sec).
> Number of exits/sec decreased 6x.
> The same improvement was shown when I tested with 3 vms running netperf
> (4086 MB/sec -> 5545 MB/sec).
> 
> filebench, 1 vm:
> ops/sec improved by 13% with the polling patch. Number of exits was reduced by
> 31%.
> The same experiment with 3 vms running filebench showed similar numbers.
> 
> Signed-off-by: Razya Ladelsky <razya@...ibm.com>
> ---
>  drivers/vhost/net.c   |    6 +-
>  drivers/vhost/scsi.c  |    6 +-
>  drivers/vhost/vhost.c |  245 +++++++++++++++++++++++++++++++++++++++++++++++--
>  drivers/vhost/vhost.h |   38 +++++++-
>  4 files changed, 277 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 971a760..558aecb 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -742,8 +742,10 @@ static int vhost_net_open(struct inode *inode, struct file *f)
>  	}
>  	vhost_dev_init(dev, vqs, VHOST_NET_VQ_MAX);
>  
> -	vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT, dev);
> -	vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN, dev);
> +	vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT,
> +			vqs[VHOST_NET_VQ_TX]);
> +	vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN,
> +			vqs[VHOST_NET_VQ_RX]);
>  
>  	f->private_data = n;
>  
> diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c
> index 4f4ffa4..665eeeb 100644
> --- a/drivers/vhost/scsi.c
> +++ b/drivers/vhost/scsi.c
> @@ -1528,9 +1528,9 @@ static int vhost_scsi_open(struct inode *inode, struct file *f)
>  	if (!vqs)
>  		goto err_vqs;
>  
> -	vhost_work_init(&vs->vs_completion_work, vhost_scsi_complete_cmd_work);
> -	vhost_work_init(&vs->vs_event_work, tcm_vhost_evt_work);
> -
> +	vhost_work_init(&vs->vs_completion_work, NULL,
> +						vhost_scsi_complete_cmd_work);
> +	vhost_work_init(&vs->vs_event_work, NULL, tcm_vhost_evt_work);
>  	vs->vs_events_nr = 0;
>  	vs->vs_events_missed = false;
>  
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index c90f437..fbe8174 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -24,9 +24,17 @@
>  #include <linux/slab.h>
>  #include <linux/kthread.h>
>  #include <linux/cgroup.h>
> +#include <linux/jiffies.h>
>  #include <linux/module.h>
>  
>  #include "vhost.h"
> +static int poll_start_rate = 0;
> +module_param(poll_start_rate, int, S_IRUGO|S_IWUSR);
> +MODULE_PARM_DESC(poll_start_rate, "Start continuous polling of virtqueue when rate of events is at least this number per jiffy. If 0, never start polling.");
> +
> +static int poll_stop_idle = 3*HZ; /* 3 seconds */
> +module_param(poll_stop_idle, int, S_IRUGO|S_IWUSR);
> +MODULE_PARM_DESC(poll_stop_idle, "Stop continuous polling of virtqueue after this many jiffies of no work.");
>  
>  enum {
>  	VHOST_MEMORY_MAX_NREGIONS = 64,
> @@ -58,27 +66,28 @@ static int vhost_poll_wakeup(wait_queue_t *wait, unsigned mode, int sync,
>  	return 0;
>  }
>  
> -void vhost_work_init(struct vhost_work *work, vhost_work_fn_t fn)
> +void vhost_work_init(struct vhost_work *work, struct vhost_virtqueue *vq,
> +							vhost_work_fn_t fn)
>  {
>  	INIT_LIST_HEAD(&work->node);
>  	work->fn = fn;
>  	init_waitqueue_head(&work->done);
>  	work->flushing = 0;
>  	work->queue_seq = work->done_seq = 0;
> +	work->vq = vq;
>  }
>  EXPORT_SYMBOL_GPL(vhost_work_init);
>  
>  /* Init poll structure */
>  void vhost_poll_init(struct vhost_poll *poll, vhost_work_fn_t fn,
> -		     unsigned long mask, struct vhost_dev *dev)
> +		     unsigned long mask, struct vhost_virtqueue *vq)
>  {
>  	init_waitqueue_func_entry(&poll->wait, vhost_poll_wakeup);
>  	init_poll_funcptr(&poll->table, vhost_poll_func);
>  	poll->mask = mask;
> -	poll->dev = dev;
> +	poll->dev = vq->dev;
>  	poll->wqh = NULL;
> -
> -	vhost_work_init(&poll->work, fn);
> +	vhost_work_init(&poll->work, vq, fn);
>  }
>  EXPORT_SYMBOL_GPL(vhost_poll_init);
>  
> @@ -174,6 +183,86 @@ void vhost_poll_queue(struct vhost_poll *poll)
>  }
>  EXPORT_SYMBOL_GPL(vhost_poll_queue);
>  
> +/* Enable or disable virtqueue polling (vqpoll.enabled) for a virtqueue.
> + *
> + * Enabling this mode it tells the guest not to notify ("kick") us when its
> + * has made more work available on this virtqueue; Rather, we will continuously
> + * poll this virtqueue in the worker thread. If multiple virtqueues are polled,
> + * the worker thread polls them all, e.g., in a round-robin fashion.
> + * Note that vqpoll.enabled doesn't always mean that this virtqueue is
> + * actually being polled: The backend (e.g., net.c) may temporarily disable it
> + * using vhost_disable/enable_notify(), while vqpoll.enabled is unchanged.
> + *
> + * It is assumed that these functions are called relatively rarely, when vhost
> + * notices that this virtqueue's usage pattern significantly changed in a way
> + * that makes polling more efficient than notification, or vice versa.
> + * Also, we assume that vhost_vq_disable_vqpoll() is always called on vq
> + * cleanup, so any allocations done by vhost_vq_enable_vqpoll() can be
> + * reclaimed.
> + */
> +static void vhost_vq_enable_vqpoll(struct vhost_virtqueue *vq)
> +{
> +	if (vq->vqpoll.enabled)
> +		return; /* already enabled, nothing to do */
> +	if (!vq->handle_kick)
> +		return; /* polling will be a waste of time if no callback! */
> +	if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY)) {
> +		/* vq has guest notifications enabled. Disable them,
> +		   and instead add vq to the polling list */
> +		vhost_disable_notify(vq->dev, vq);
> +		list_add_tail(&vq->vqpoll.link, &vq->dev->vqpoll_list);
> +	}
> +	vq->vqpoll.jiffies_last_kick = jiffies;
> +	__get_user(vq->avail_idx, &vq->avail->idx);
> +	vq->vqpoll.enabled = true;
> +
> +	/* Map userspace's vq->avail to the kernel's memory space. */
> +	if (get_user_pages_fast((unsigned long)vq->avail, 1, 0,
> +		&vq->vqpoll.avail_page) != 1) {
> +		/* TODO: can this happen, as we check access
> +		to vq->avail in advance? */
> +		BUG();
> +	}
> +	vq->vqpoll.avail_mapped = (struct vring_avail *) (
> +		(unsigned long)kmap(vq->vqpoll.avail_page) |
> +		((unsigned long)vq->avail & ~PAGE_MASK));
> +}
> +
> +/*
> + * This function doesn't always succeed in changing the mode. Sometimes
> + * a temporary race condition prevents turning on guest notifications, so
> + * vq should be polled next time again.
> + */
> +static void vhost_vq_disable_vqpoll(struct vhost_virtqueue *vq)
> +{
> +	if (!vq->vqpoll.enabled)
> +		return; /* already disabled, nothing to do */
> +
> +	vq->vqpoll.enabled = false;
> +
> +	if (!list_empty(&vq->vqpoll.link)) {
> +		/* vq is on the polling list, remove it from this list and
> +		 * instead enable guest notifications. */
> +		list_del_init(&vq->vqpoll.link);
> +		if (unlikely(vhost_enable_notify(vq->dev, vq))
> +			&& !vq->vqpoll.shutdown) {
> +			/* Race condition: guest wrote before we enabled
> +			 * notification, so we'll never get a notification for
> +			 * this work - so continue polling mode for a while. */
> +			vhost_disable_notify(vq->dev, vq);
> +			vq->vqpoll.enabled = true;
> +			vhost_enable_notify(vq->dev, vq);
> +			return;
> +		}
> +	}
> +
> +	if (vq->vqpoll.avail_mapped) {
> +		kunmap(vq->vqpoll.avail_page);
> +		put_page(vq->vqpoll.avail_page);
> +		vq->vqpoll.avail_mapped = 0;
> +	}
> +}
> +
>  static void vhost_vq_reset(struct vhost_dev *dev,
>  			   struct vhost_virtqueue *vq)
>  {
> @@ -199,6 +288,48 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>  	vq->call = NULL;
>  	vq->log_ctx = NULL;
>  	vq->memory = NULL;
> +	INIT_LIST_HEAD(&vq->vqpoll.link);
> +	vq->vqpoll.enabled = false;
> +	vq->vqpoll.shutdown = false;
> +	vq->vqpoll.avail_mapped = NULL;
> +}
> +
> +/* roundrobin_poll() takes worker->vqpoll_list, and returns one of the
> + * virtqueues which the caller should kick, or NULL in case none should be
> + * kicked. roundrobin_poll() also disables polling on a virtqueue which has
> + * been polled for too long without success.
> + *
> + * This current implementation (the "round-robin" implementation) only
> + * polls the first vq in the list, returning it or NULL as appropriate, and
> + * moves this vq to the end of the list, so next time a different one is
> + * polled.
> + */
> +static struct vhost_virtqueue *roundrobin_poll(struct list_head *list)
> +{
> +	struct vhost_virtqueue *vq;
> +	u16 avail_idx;
> +
> +	if (list_empty(list))
> +		return NULL;
> +
> +	vq = list_first_entry(list, struct vhost_virtqueue, vqpoll.link);
> +	WARN_ON(!vq->vqpoll.enabled);
> +	list_move_tail(&vq->vqpoll.link, list);
> +
> +	/* See if there is any new work available from the guest. */
> +	/* TODO: can check the optional idx feature, and if we haven't
> +	* reached that idx yet, don't kick... */
> +	avail_idx = vq->vqpoll.avail_mapped->idx;
> +	if (avail_idx != vq->last_avail_idx)
> +		return vq;
> +
> +	if (jiffies > vq->vqpoll.jiffies_last_kick + poll_stop_idle) {
> +		/* We've been polling this virtqueue for a long time with no
> +		* results, so switch back to guest notification
> +		*/
> +		vhost_vq_disable_vqpoll(vq);
> +	}
> +	return NULL;
>  }
>  
>  static int vhost_worker(void *data)
> @@ -237,12 +368,62 @@ static int vhost_worker(void *data)
>  		spin_unlock_irq(&dev->work_lock);
>  
>  		if (work) {
> +			struct vhost_virtqueue *vq = work->vq;
>  			__set_current_state(TASK_RUNNING);
>  			work->fn(work);
> +			/* Keep track of the work rate, for deciding when to
> +			 * enable polling */
> +			if (vq) {
> +				if (vq->vqpoll.jiffies_last_work != jiffies) {
> +					vq->vqpoll.jiffies_last_work = jiffies;
> +					vq->vqpoll.work_this_jiffy = 0;
> +				}
> +				vq->vqpoll.work_this_jiffy++;
> +			}
> +			/* If vq is in the round-robin list of virtqueues being
> +			 * constantly checked by this thread, move vq the end
> +			 * of the queue, because it had its fair chance now.
> +			 */
> +			if (vq && !list_empty(&vq->vqpoll.link)) {
> +				list_move_tail(&vq->vqpoll.link,
> +					&dev->vqpoll_list);
> +			}
> +			/* Otherwise, if this vq is looking for notifications
> +			 * but vq polling is not enabled for it, do it now.
> +			 */
> +			else if (poll_start_rate && vq && vq->handle_kick &&
> +				!vq->vqpoll.enabled &&
> +				!vq->vqpoll.shutdown &&
> +				!(vq->used_flags & VRING_USED_F_NO_NOTIFY) &&
> +				vq->vqpoll.work_this_jiffy >=
> +					poll_start_rate) {
> +				vhost_vq_enable_vqpoll(vq);
> +			}
> +		}
> +		/* Check one virtqueue from the round-robin list */
> +		if (!list_empty(&dev->vqpoll_list)) {
> +			struct vhost_virtqueue *vq;
> +
> +			vq = roundrobin_poll(&dev->vqpoll_list);
> +
> +			if (vq) {
> +				vq->handle_kick(&vq->poll.work);
> +				vq->vqpoll.jiffies_last_kick = jiffies;
> +			}
> +
> +			/* If our polling list isn't empty, ask to continue
> +			 * running this thread, don't yield.
> +			 */
> +			__set_current_state(TASK_RUNNING);
>  			if (need_resched())
>  				schedule();
> -		} else
> -			schedule();
> +		} else {
> +			if (work) {
> +				if (need_resched())
> +					schedule();
> +			} else
> +				schedule();
> +		}
>  
>  	}
>  	unuse_mm(dev->mm);
> @@ -306,6 +487,7 @@ void vhost_dev_init(struct vhost_dev *dev,
>  	dev->mm = NULL;
>  	spin_lock_init(&dev->work_lock);
>  	INIT_LIST_HEAD(&dev->work_list);
> +	INIT_LIST_HEAD(&dev->vqpoll_list);
>  	dev->worker = NULL;
>  
>  	for (i = 0; i < dev->nvqs; ++i) {
> @@ -318,7 +500,7 @@ void vhost_dev_init(struct vhost_dev *dev,
>  		vhost_vq_reset(dev, vq);
>  		if (vq->handle_kick)
>  			vhost_poll_init(&vq->poll, vq->handle_kick,
> -					POLLIN, dev);
> +					POLLIN, vq);
>  	}
>  }
>  EXPORT_SYMBOL_GPL(vhost_dev_init);
> @@ -350,7 +532,7 @@ static int vhost_attach_cgroups(struct vhost_dev *dev)
>  	struct vhost_attach_cgroups_struct attach;
>  
>  	attach.owner = current;
> -	vhost_work_init(&attach.work, vhost_attach_cgroups_work);
> +	vhost_work_init(&attach.work, NULL, vhost_attach_cgroups_work);
>  	vhost_work_queue(dev, &attach.work);
>  	vhost_work_flush(dev, &attach.work);
>  	return attach.ret;
> @@ -444,6 +626,26 @@ void vhost_dev_stop(struct vhost_dev *dev)
>  }
>  EXPORT_SYMBOL_GPL(vhost_dev_stop);
>  
> +/* shutdown_vqpoll() asks the worker thread to shut down virtqueue polling
> + * mode for a given virtqueue which is itself being shut down. We ask the
> + * worker thread to do this rather than doing it directly, so that we don't
> + * race with the worker thread's use of the queue.
> + */
> +static void shutdown_vqpoll_work(struct vhost_work *work)
> +{
> +	work->vq->vqpoll.shutdown = true;
> +	vhost_vq_disable_vqpoll(work->vq);
> +	WARN_ON(work->vq->vqpoll.avail_mapped);
> +}
> +
> +static void shutdown_vqpoll(struct vhost_virtqueue *vq)
> +{
> +	struct vhost_work work;
> +
> +	vhost_work_init(&work, vq, shutdown_vqpoll_work);
> +	vhost_work_queue(vq->dev, &work);
> +	vhost_work_flush(vq->dev, &work);
> +}
>  /* Caller should have device mutex if and only if locked is set */
>  void vhost_dev_cleanup(struct vhost_dev *dev, bool locked)
>  {
> @@ -460,6 +662,7 @@ void vhost_dev_cleanup(struct vhost_dev *dev, bool locked)
>  			eventfd_ctx_put(dev->vqs[i]->call_ctx);
>  		if (dev->vqs[i]->call)
>  			fput(dev->vqs[i]->call);
> +		shutdown_vqpoll(dev->vqs[i]);
>  		vhost_vq_reset(dev, dev->vqs[i]);
>  	}
>  	vhost_dev_free_iovecs(dev);
> @@ -1491,6 +1694,19 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>  	u16 avail_idx;
>  	int r;
>  
> +	/* In polling mode, when the backend (e.g., net.c) asks to enable
> +	 * notifications, we don't enable guest notifications. Instead, start
> +	 * polling on this vq by adding it to the round-robin list.
> +	 */
> +	if (vq->vqpoll.enabled) {
> +		if (list_empty(&vq->vqpoll.link)) {
> +			list_add_tail(&vq->vqpoll.link,
> +				&vq->dev->vqpoll_list);
> +			vq->vqpoll.jiffies_last_kick = jiffies;
> +		}
> +		return false;
> +	}
> +
>  	if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
>  		return false;
>  	vq->used_flags &= ~VRING_USED_F_NO_NOTIFY;
> @@ -1528,6 +1744,17 @@ void vhost_disable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>  {
>  	int r;
>  
> +	/* If this virtqueue is vqpoll.enabled, and on the polling list, it
> +	 * will generate notifications even if the guest is asked not to send
> +	 * them. So we must remove it from the round-robin polling list.
> +	 * Note that vqpoll.enabled remains set.
> +	 */
> +	if (vq->vqpoll.enabled) {
> +		if (!list_empty(&vq->vqpoll.link))
> +			list_del_init(&vq->vqpoll.link);
> +		return;
> +	}
> +
>  	if (vq->used_flags & VRING_USED_F_NO_NOTIFY)
>  		return;
>  	vq->used_flags |= VRING_USED_F_NO_NOTIFY;
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 3eda654..11aaaf4 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -24,6 +24,7 @@ struct vhost_work {
>  	int			  flushing;
>  	unsigned		  queue_seq;
>  	unsigned		  done_seq;
> +	struct vhost_virtqueue    *vq;
>  };
>  
>  /* Poll a file (eventfd or socket) */
> @@ -37,11 +38,12 @@ struct vhost_poll {
>  	struct vhost_dev	 *dev;
>  };
>  
> -void vhost_work_init(struct vhost_work *work, vhost_work_fn_t fn);
> +void vhost_work_init(struct vhost_work *work, struct vhost_virtqueue *vq,
> +							vhost_work_fn_t fn);
>  void vhost_work_queue(struct vhost_dev *dev, struct vhost_work *work);
>  
>  void vhost_poll_init(struct vhost_poll *poll, vhost_work_fn_t fn,
> -		     unsigned long mask, struct vhost_dev *dev);
> +		     unsigned long mask, struct vhost_virtqueue  *vq);
>  int vhost_poll_start(struct vhost_poll *poll, struct file *file);
>  void vhost_poll_stop(struct vhost_poll *poll);
>  void vhost_poll_flush(struct vhost_poll *poll);
> @@ -54,8 +56,6 @@ struct vhost_log {
>  	u64 len;
>  };
>  
> -struct vhost_virtqueue;
> -
>  /* The virtqueue structure describes a queue attached to a device. */
>  struct vhost_virtqueue {
>  	struct vhost_dev *dev;
> @@ -110,6 +110,35 @@ struct vhost_virtqueue {
>  	/* Log write descriptors */
>  	void __user *log_base;
>  	struct vhost_log *log;
> +	struct {
> +      /* When a virtqueue is in vqpoll.enabled mode, it declares
> +       * that instead of using guest notifications (kicks) to
> +       * discover new work, we prefer to continuously poll this
> +       * virtqueue in the worker thread.
> +       * If !enabled, the rest of the fields below are undefined.
> +       */
> +		bool enabled;
> +      /* vqpoll.enabled doesn't always mean that this virtqueue is
> +       * actually being polled: The backend (e.g., net.c) may
> +       * temporarily disable it using vhost_disable/enable_notify().
> +       * vqpoll.link is used to maintain the thread's round-robin
> +       * list of virtqueues that actually need to be polled.
> +       * Note list_empty(link) means this virtqueue isn't polled.
> +       */
> +		struct list_head link;
> +      /* If this flag is true, the virtqueue is being shut down,
> +       * so vqpoll should not be re-enabled.
> +       */
> +		bool shutdown;
> +      /* Various counters used to decide when to enter polling mode
> +       * or leave it and return to notification mode.
> +       */
> +		unsigned long jiffies_last_kick;
> +		unsigned long jiffies_last_work;
> +		int work_this_jiffy;
> +		struct page *avail_page;
> +		volatile struct vring_avail *avail_mapped;
> +	} vqpoll;
>  };
>  
>  struct vhost_dev {
> @@ -123,6 +152,7 @@ struct vhost_dev {
>  	spinlock_t work_lock;
>  	struct list_head work_list;
>  	struct task_struct *worker;
> +	struct list_head vqpoll_list;
>  };
>  
>  void vhost_dev_init(struct vhost_dev *, struct vhost_virtqueue **vqs, int nvqs);
> -- 
> 1.7.9.5
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