[PATCH v5 0/5] vhost/vsock: add support for VHOST_RESET_OWNER and CPR migration

[PATCH v5 0/5] vhost/vsock: add support for VHOST_RESET_OWNER and CPR migration

[Andrey Drobyshev]

v4 -> v5:

  * Patch 4:
    - call synchronize_rcu() unconditionally;
    - call vhost_dev_flush(dev) instead of vhost_run_work_list(worker)
      to drain remaining work;
    - reword comment and commit message.

v4: https://lore.kernel.org/virtualization/20260714151638.143019-1-andrey.drobyshev@virtuozzo.com

Andrey Drobyshev (3):
  vhost/vsock: suppress EHOSTUNREACH fast-fail during CPR pause
  vhost/vsock: re-scan TX virtqueue on device start
  vhost: synchronize with RCU readers when freeing workers

Pavel Tikhomirov (2):
  vhost/vsock: split out vhost_vsock_drop_backends helper
  vhost/vsock: add VHOST_RESET_OWNER ioctl

 drivers/vhost/vhost.c | 11 ++++++
 drivers/vhost/vsock.c | 80 +++++++++++++++++++++++++++++++++----------
 2 files changed, 72 insertions(+), 19 deletions(-)

-- 
2.47.1

[PATCH v5 1/5] vhost/vsock: split out vhost_vsock_drop_backends helper

[Andrey Drobyshev]

From: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>

Split the actual backend dropping part from vhost_vsock_stop.  We're
going to need it for the VHOST_RESET_OWNER implementation in the
following patch, when vsock->dev.mutex is already taken and owner is
checked.

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Reviewed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
---
 drivers/vhost/vsock.c | 26 +++++++++++++++++---------
 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index 9aaab6bb8061..b12221ce6faf 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c
@@ -664,9 +664,24 @@ static int vhost_vsock_start(struct vhost_vsock *vsock)
 	return ret;
 }
 
-static int vhost_vsock_stop(struct vhost_vsock *vsock, bool check_owner)
+static void vhost_vsock_drop_backends(struct vhost_vsock *vsock)
 {
+	struct vhost_virtqueue *vq;
 	size_t i;
+
+	lockdep_assert_held(&vsock->dev.mutex);
+
+	for (i = 0; i < ARRAY_SIZE(vsock->vqs); i++) {
+		vq = &vsock->vqs[i];
+
+		mutex_lock(&vq->mutex);
+		vhost_vq_set_backend(vq, NULL);
+		mutex_unlock(&vq->mutex);
+	}
+}
+
+static int vhost_vsock_stop(struct vhost_vsock *vsock, bool check_owner)
+{
 	int ret = 0;
 
 	mutex_lock(&vsock->dev.mutex);
@@ -677,14 +692,7 @@ static int vhost_vsock_stop(struct vhost_vsock *vsock, bool check_owner)
 			goto err;
 	}
 
-	for (i = 0; i < ARRAY_SIZE(vsock->vqs); i++) {
-		struct vhost_virtqueue *vq = &vsock->vqs[i];
-
-		mutex_lock(&vq->mutex);
-		vhost_vq_set_backend(vq, NULL);
-		mutex_unlock(&vq->mutex);
-	}
-
+	vhost_vsock_drop_backends(vsock);
 err:
 	mutex_unlock(&vsock->dev.mutex);
 	return ret;
-- 
2.47.1

[PATCH v5 2/5] vhost/vsock: suppress EHOSTUNREACH fast-fail during CPR pause

[Andrey Drobyshev]

Earlier commit bb26ed5f3a8b ("vhost/vsock: Refuse the connection
immediately when guest isn't ready") added a fast-fail in
vhost_transport_send_pkt().  It rejects every host send with -EHOSTUNREACH
until the destination calls SET_RUNNING(1).  The fast-fail condition checks
whether device's backends are dropped, and if they're, the guest is
considered to be not ready.

However, there might be other reasons for backends to be nulled.  In
particular, when QEMU is performing CPR (checkpoint-restore) migration,
device ownership is being RESET and SET again, which leads to backends
drop and reattach.  If we end up connecting during this window, an
AF_VSOCK client gets -EHOSTUNREACH, which is wrong.

Add an 'ever_started' flag which is set once in vhost_vsock_start() and is
never cleared.  The behaviour changes to:

  * When device was never started -> flag is unset -> no listener can
    exist yet -> fast-fail;
  * Once the device starts -> flag is set -> we don't fast-fail ->
    we queue and preserve during any later stop / CPR pause.

The VHOST_RESET_OWNER ioctl is implemented in a following patch, and
without RESET_OWNER the problem we fix here isn't manifesting - thus
this patch is a preparation to support RESET_OWNER.

Important caveat: after the first start, a connect during any stopped
window is queued instead of fast-failed.  That was the behaviour before
the patch bb26ed5f3a8b, and we're restoring it now.  However we still
keep the behaviour originally intended by that commit (i.e. fast-fail if
there's no real listener yet) while fixing the CPR path.

Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Reviewed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
---
 drivers/vhost/vsock.c | 22 ++++++++++++----------
 1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index b12221ce6faf..27169a09e87e 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c
@@ -61,6 +61,7 @@ struct vhost_vsock {
 
 	u32 guest_cid;
 	bool seqpacket_allow;
+	bool ever_started; /* set on first SET_RUNNING(1); never cleared */
 };
 
 static u32 vhost_transport_get_local_cid(void)
@@ -302,17 +303,12 @@ vhost_transport_send_pkt(struct sk_buff *skb, struct net *net)
 		return -ENODEV;
 	}
 
-	/* Fast-fail if the guest hasn't enabled the RX vq yet. Queuing the packet
-	 * and making the caller wait is pointless: even if the guest manages to init
-	 * within the timeout, it'll immediately reply with RST, because there's no
-	 * listener on the port yet.
-	 *
-	 * vhost_vq_get_backend() without vq->mutex is acceptable here: locking
-	 * the mutex would be too expensive in this hot path, and we already have
-	 * all the outcomes covered: if the backend becomes NULL right after the check,
-	 * vhost_transport_do_send_pkt() will check it under the mutex anyway.
+	/* Fast-fail until the guest first enables the device (SET_RUNNING(1)).
+	 * Before that there is no listener, so queuing is pointless.
+	 * 'ever_started' is never cleared, so once we're up we keep queuing
+	 * across later stop / CPR-pause windows.
 	 */
-	if (unlikely(!data_race(vhost_vq_get_backend(&vsock->vqs[VSOCK_VQ_RX])))) {
+	if (unlikely(!READ_ONCE(vsock->ever_started))) {
 		rcu_read_unlock();
 		kfree_skb(skb);
 		return -EHOSTUNREACH;
@@ -640,6 +636,11 @@ static int vhost_vsock_start(struct vhost_vsock *vsock)
 		mutex_unlock(&vq->mutex);
 	}
 
+	/* Set 'ever_started' flag on the first start; never cleared, so send_pkt
+	 * keeps queuing (instead of fast-failing) on later stop / CPR pauses.
+	 */
+	WRITE_ONCE(vsock->ever_started, true);
+
 	/* Some packets may have been queued before the device was started,
 	 * let's kick the send worker to send them.
 	 */
@@ -728,6 +729,7 @@ static int vhost_vsock_dev_open(struct inode *inode, struct file *file)
 
 	vsock->guest_cid = 0; /* no CID assigned yet */
 	vsock->seqpacket_allow = false;
+	vsock->ever_started = false;
 
 	atomic_set(&vsock->queued_replies, 0);
 
-- 
2.47.1

[PATCH v5 3/5] vhost/vsock: re-scan TX virtqueue on device start

[Andrey Drobyshev]

During QEMU CPR live-update (and VHOST_RESET_OWNER in general) the guest
keeps running while the host drops and later re-attaches vhost backends.
If the guest adds a buffer to the TX virtqueue (guest->host) and kicks
while the backend is temporarily NULL (between vhost_vsock_drop_backends()
and the next vhost_vsock_start()), then the kick is delivered to the
vhost worker, handle_tx_kick() sees a NULL backend and returns, and the
kick signal is consumed.  The buffer is then left in the ring.

Then upon device start vhost_vsock_start() only re-kicks the RX send
worker, never the TX VQ, so the buffer is processed only if the guest
happens to kick again.  But if the guest itself is now waiting for data
from the host, it will never kick TX VQ again, and we end up in a
deadlock.

The issue itself is pre-existing, but it only manifests during a device
pause caused by VHOST_RESET_OWNER.  Namely, the deadlock is reproduced
during active host->guest socat data transfer under multiple consecutive
CPR live-update's.

To fix this, in vhost_vsock_start(), after kicking the RX send worker, also
queue the TX vq poll so any buffers the guest enqueued while we were paused
get scanned.

The VHOST_RESET_OWNER ioctl itself is implemented in the following
patch, thus this patch is a preparation to support VHOST_RESET_OWNER.

Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Reviewed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
---
 drivers/vhost/vsock.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index 27169a09e87e..d5022d21120b 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c
@@ -646,6 +646,13 @@ static int vhost_vsock_start(struct vhost_vsock *vsock)
 	 */
 	vhost_vq_work_queue(&vsock->vqs[VSOCK_VQ_RX], &vsock->send_pkt_work);
 
+	/* The guest may have added TX buffers while the device was stopped
+	 * (e.g. across VHOST_RESET_OWNER) and their kicks got consumed by
+	 * the NULL-backend window.  Re-scan the TX VQ, mirroring the RX
+	 * send-worker kick above.
+	 */
+	vhost_poll_queue(&vsock->vqs[VSOCK_VQ_TX].poll);
+
 	mutex_unlock(&vsock->dev.mutex);
 	return 0;
 
-- 
2.47.1

[PATCH v5 4/5] vhost: synchronize with RCU readers when freeing workers

[Andrey Drobyshev]

vhost_vq_work_queue() only holds the RCU read lock while it dereferences
vq->worker and queues work on it.  vhost_workers_free() however clears
the vq->worker pointers and immediately frees the workers, without
waiting for a grace period.  A caller that fetched the worker right
before the pointer was cleared can therefore still be queueing work on
it while it is freed.  And even when the queueing itself wins the race,
the work is never run, so its VHOST_WORK_QUEUED bit stays set and all
future attempts to queue it are silently skipped.

None of the current callers can actually hit this: net and scsi stop
their virtqueues before the workers are freed, and vsock unhashes the
device and does synchronize_rcu() of its own in vhost_vsock_dev_release()
before the workers go away.  But the upcoming VHOST_RESET_OWNER support
in vhost-vsock keeps the device hashed while its workers are freed, so
the lockless send/cancel paths become able to race with the teardown.

Fix this by clearing the vq->worker pointers, waiting for a grace
period, and then flushing the workers so any work the last readers
queued runs before the workers are freed.

Fixes: 228a27cf78af ("vhost: Allow worker switching while work is queueing")
Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
---
 drivers/vhost/vhost.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 4c525b3e16ea..d6e235c25254 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -729,6 +729,17 @@ static void vhost_workers_free(struct vhost_dev *dev)
 
 	for (i = 0; i < dev->nvqs; i++)
 		rcu_assign_pointer(dev->vqs[i]->worker, NULL);
+
+	/*
+	 * vhost_vq_work_queue() reads vq->worker under rcu_read_lock(), so a
+	 * reader that fetched a worker before we cleared the pointers above
+	 * may still be queueing work on it.  Wait for those readers to
+	 * finish, then flush so any work they queued runs (clearing
+	 * VHOST_WORK_QUEUED) before the workers are freed.
+	 */
+	synchronize_rcu();
+	vhost_dev_flush(dev);
+
 	/*
 	 * Free the default worker we created and cleanup workers userspace
 	 * created but couldn't clean up (it forgot or crashed).
-- 
2.47.1

[PATCH v5 5/5] vhost/vsock: add VHOST_RESET_OWNER ioctl

[Andrey Drobyshev]

From: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>

This ioctl is needed for QEMU's CPR (checkpoint-restore) migration of
the guest with vhost-vsock device.  For this to work, we need to reset
the device ownership on the source side by calling RESET_OWNER, and then
claim it on the dest side by calling SET_OWNER.  We expect not to lose any
AF_VSOCK connection while this happens.

To that end, unlike the release path, RESET_OWNER keeps the guest CID
hashed: established connections survive, and host sends issued while
the device is between owners simply stay on send_pkt_queue until the
next device start drains them.

Since the device stays reachable through the CID hash, the lockless
send/cancel paths can race with the worker teardown in
vhost_workers_free().  The previous commit ("vhost: synchronize with
RCU readers when freeing workers") makes that safe.

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
---
 drivers/vhost/vsock.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index d5022d21120b..86f25ff80722 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c
@@ -903,6 +903,29 @@ static int vhost_vsock_set_features(struct vhost_vsock *vsock, u64 features)
 	return -EFAULT;
 }
 
+static long vhost_vsock_reset_owner(struct vhost_vsock *vsock)
+{
+	struct vhost_iotlb *umem;
+	long err;
+
+	mutex_lock(&vsock->dev.mutex);
+	err = vhost_dev_check_owner(&vsock->dev);
+	if (err)
+		goto done;
+	umem = vhost_dev_reset_owner_prepare();
+	if (!umem) {
+		err = -ENOMEM;
+		goto done;
+	}
+	vhost_vsock_drop_backends(vsock);
+	vhost_vsock_flush(vsock);
+	vhost_dev_stop(&vsock->dev);
+	vhost_dev_reset_owner(&vsock->dev, umem);
+done:
+	mutex_unlock(&vsock->dev.mutex);
+	return err;
+}
+
 static long vhost_vsock_dev_ioctl(struct file *f, unsigned int ioctl,
 				  unsigned long arg)
 {
@@ -946,6 +969,8 @@ static long vhost_vsock_dev_ioctl(struct file *f, unsigned int ioctl,
 			return -EOPNOTSUPP;
 		vhost_set_backend_features(&vsock->dev, features);
 		return 0;
+	case VHOST_RESET_OWNER:
+		return vhost_vsock_reset_owner(vsock);
 	default:
 		mutex_lock(&vsock->dev.mutex);
 		r = vhost_dev_ioctl(&vsock->dev, ioctl, argp);
-- 
2.47.1