Re: vhost changes (batched) in linux-next after 12/13 trigger random crashes in KVM guests after reboot

["Eugenio Pérez" <eperezma@redhat.com>]

On Fri, 2020-02-14 at 08:47 +0100, Christian Borntraeger wrote:
> repro
> 
> 
> On 14.02.20 08:43, Christian Borntraeger wrote:
> > 
> > On 14.02.20 08:40, Eugenio Perez Martin wrote:
> > > Hi.
> > > 
> > > Were the vhost and vhost_net modules loaded with dyndbg='+plt'? I miss
> > > all the others regular debug traces on that one.
> > 
> > I did 
> > 
> >  echo -n 'file drivers/vhost/vhost.c +plt' > control
> > and
> > echo -n 'file drivers/vhost/net.c +plt'  > control
> > 
> > but apparently it did not work...me hates dynamic debug.

Sorry about use dyndbg, but it is the easiest way to obtain accurate line numbers, threads id...

I usually load the module with that option (modprobe vhost dyndbg=+pfmlt && modprobe vhost_net dyndbg=+pfmlt), so it is
available from the very beginning.

> > 
> > > Thanks!
> > > 
> > > On Fri, Feb 14, 2020 at 8:34 AM Christian Borntraeger
> > > <borntraeger@de.ibm.com> wrote:
> > > > I did
> > > > ping -c 20 -f ... ; reboot
> > > > twice
> > > > 
> > > > The ping after the first reboot showed .......E
> > > > 
> > > > this was on the host console
> > > > 
> > > > [   55.951885] CPU: 34 PID: 1908 Comm: CPU 0/KVM Not tainted 5.5.0+ #21
> > > > [   55.951891] Hardware name: IBM 3906 M04 704 (LPAR)
> > > > [   55.951892] Call Trace:
> > > > [   55.951902]  [<0000001ede114132>] show_stack+0x8a/0xd0
> > > > [   55.951906]  [<0000001edeb0672a>] dump_stack+0x8a/0xb8
> > > > [   55.951915]  [<000003ff803736a6>] vhost_vring_ioctl+0x6fe/0x858 [vhost]
> > > > [   55.951919]  [<000003ff8042a608>] vhost_net_ioctl+0x510/0x570 [vhost_net]
> > > > [   55.951924]  [<0000001ede3c4dd8>] do_vfs_ioctl+0x430/0x6f8
> > > > [   55.951926]  [<0000001ede3c5124>] ksys_ioctl+0x84/0xb0
> > > > [   55.951927]  [<0000001ede3c51ba>] __s390x_sys_ioctl+0x2a/0x38
> > > > [   55.951931]  [<0000001edeb27f72>] system_call+0x2a6/0x2c8
> > > > [   55.951949] CPU: 34 PID: 1908 Comm: CPU 0/KVM Not tainted 5.5.0+ #21
> > > > [   55.951950] Hardware name: IBM 3906 M04 704 (LPAR)
> > > > [   55.951951] Call Trace:
> > > > [   55.951952]  [<0000001ede114132>] show_stack+0x8a/0xd0
> > > > [   55.951954]  [<0000001edeb0672a>] dump_stack+0x8a/0xb8
> > > > [   55.951956]  [<000003ff803736a6>] vhost_vring_ioctl+0x6fe/0x858 [vhost]
> > > > [   55.951958]  [<000003ff8042a608>] vhost_net_ioctl+0x510/0x570 [vhost_net]
> > > > [   55.951959]  [<0000001ede3c4dd8>] do_vfs_ioctl+0x430/0x6f8
> > > > [   55.951961]  [<0000001ede3c5124>] ksys_ioctl+0x84/0xb0
> > > > [   55.951962]  [<0000001ede3c51ba>] __s390x_sys_ioctl+0x2a/0x38
> > > > [   55.951964]  [<0000001edeb27f72>] system_call+0x2a6/0x2c8
> > > > [   55.951997] Guest moved vq 0000000063d896c6 used index from 44 to 0
> > > > [   56.609831] unexpected descriptor format for RX: out 0, in 0
> > > > [   86.540460] CPU: 6 PID: 1908 Comm: CPU 0/KVM Not tainted 5.5.0+ #21
> > > > [   86.540464] Hardware name: IBM 3906 M04 704 (LPAR)
> > > > [   86.540466] Call Trace:
> > > > [   86.540473]  [<0000001ede114132>] show_stack+0x8a/0xd0
> > > > [   86.540477]  [<0000001edeb0672a>] dump_stack+0x8a/0xb8
> > > > [   86.540486]  [<000003ff803736a6>] vhost_vring_ioctl+0x6fe/0x858 [vhost]
> > > > [   86.540490]  [<000003ff8042a608>] vhost_net_ioctl+0x510/0x570 [vhost_net]
> > > > [   86.540494]  [<0000001ede3c4dd8>] do_vfs_ioctl+0x430/0x6f8
> > > > [   86.540496]  [<0000001ede3c5124>] ksys_ioctl+0x84/0xb0
> > > > [   86.540498]  [<0000001ede3c51ba>] __s390x_sys_ioctl+0x2a/0x38
> > > > [   86.540501]  [<0000001edeb27f72>] system_call+0x2a6/0x2c8
> > > > [   86.540524] CPU: 6 PID: 1908 Comm: CPU 0/KVM Not tainted 5.5.0+ #21
> > > > [   86.540525] Hardware name: IBM 3906 M04 704 (LPAR)
> > > > [   86.540526] Call Trace:
> > > > [   86.540527]  [<0000001ede114132>] show_stack+0x8a/0xd0
> > > > [   86.540528]  [<0000001edeb0672a>] dump_stack+0x8a/0xb8
> > > > [   86.540531]  [<000003ff803736a6>] vhost_vring_ioctl+0x6fe/0x858 [vhost]
> > > > [   86.540532]  [<000003ff8042a608>] vhost_net_ioctl+0x510/0x570 [vhost_net]
> > > > [   86.540534]  [<0000001ede3c4dd8>] do_vfs_ioctl+0x430/0x6f8
> > > > [   86.540536]  [<0000001ede3c5124>] ksys_ioctl+0x84/0xb0
> > > > [   86.540537]  [<0000001ede3c51ba>] __s390x_sys_ioctl+0x2a/0x38
> > > > [   86.540538]  [<0000001edeb27f72>] system_call+0x2a6/0x2c8
> > > > [   86.540570] unexpected descriptor format for RX: out 0, in 0
> > > > [   86.540577] Unexpected header len for TX: 0 expected 0
> > > > 
> > > > 
> > > > On 14.02.20 08:06, Eugenio Pérez wrote:
> > > > > Hi Christian.
> > > > > 
> > > > > Sorry, that was meant to be applied over previous debug patch.
> > > > > 
> > > > > Here I inline the one meant to be applied over eccb852f1fe6bede630e2e4f1a121a81e34354ab.
> > > > > 
> > > > > Thanks!
> > > > > 
> > > > > From d978ace99e4844b49b794d768385db3d128a4cc0 Mon Sep 17 00:00:00 2001
> > > > > From: =?UTF-8?q?Eugenio=20P=C3=A9rez?= <eperezma@redhat.com>
> > > > > Date: Fri, 14 Feb 2020 08:02:26 +0100
> > > > > Subject: [PATCH] vhost: disable all features and trace last_avail_idx and
> > > > >  ioctl calls
> > > > > 
> > > > > ---
> > > > >  drivers/vhost/net.c   | 20 +++++++++++++++++---
> > > > >  drivers/vhost/vhost.c | 25 +++++++++++++++++++++++--
> > > > >  drivers/vhost/vhost.h | 10 +++++-----
> > > > >  3 files changed, 45 insertions(+), 10 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > > > > index e158159671fa..e4d5f843f9c0 100644
> > > > > --- a/drivers/vhost/net.c
> > > > > +++ b/drivers/vhost/net.c
> > > > > @@ -1505,10 +1505,13 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
> > > > > 
> > > > >       mutex_lock(&n->dev.mutex);
> > > > >       r = vhost_dev_check_owner(&n->dev);
> > > > > -     if (r)
> > > > > +     if (r) {
> > > > > +             pr_debug("vhost_dev_check_owner index=%u fd=%d rc r=%d", index, fd, r);
> > > > >               goto err;
> > > > > +     }
> > > > > 
> > > > >       if (index >= VHOST_NET_VQ_MAX) {
> > > > > +             pr_debug("vhost_dev_check_owner index=%u fd=%d MAX=%d", index, fd, VHOST_NET_VQ_MAX);
> > > > >               r = -ENOBUFS;
> > > > >               goto err;
> > > > >       }
> > > > > @@ -1518,22 +1521,26 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
> > > > > 
> > > > >       /* Verify that ring has been setup correctly. */
> > > > >       if (!vhost_vq_access_ok(vq)) {
> > > > > +             pr_debug("vhost_net_set_backend index=%u fd=%d !vhost_vq_access_ok", index, fd);
> > > > >               r = -EFAULT;
> > > > >               goto err_vq;
> > > > >       }
> > > > >       sock = get_socket(fd);
> > > > >       if (IS_ERR(sock)) {
> > > > >               r = PTR_ERR(sock);
> > > > > +             pr_debug("vhost_net_set_backend index=%u fd=%d get_socket err r=%d", index, fd, r);
> > > > >               goto err_vq;
> > > > >       }
> > > > > 
> > > > >       /* start polling new socket */
> > > > >       oldsock = vq->private_data;
> > > > >       if (sock != oldsock) {
> > > > > +             pr_debug("sock=%p != oldsock=%p index=%u fd=%d vq=%p", sock, oldsock, index, fd, vq);
> > > > >               ubufs = vhost_net_ubuf_alloc(vq,
> > > > >                                            sock && vhost_sock_zcopy(sock));
> > > > >               if (IS_ERR(ubufs)) {
> > > > >                       r = PTR_ERR(ubufs);
> > > > > +                     pr_debug("ubufs index=%u fd=%d err r=%d vq=%p", index, fd, r, vq);
> > > > >                       goto err_ubufs;
> > > > >               }
> > > > > 
> > > > > @@ -1541,11 +1548,15 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
> > > > >               vq->private_data = sock;
> > > > >               vhost_net_buf_unproduce(nvq);
> > > > >               r = vhost_vq_init_access(vq);
> > > > > -             if (r)
> > > > > +             if (r) {
> > > > > +                     pr_debug("init_access index=%u fd=%d r=%d vq=%p", index, fd, r, vq);
> > > > >                       goto err_used;
> > > > > +             }
> > > > >               r = vhost_net_enable_vq(n, vq);
> > > > > -             if (r)
> > > > > +             if (r) {
> > > > > +                     pr_debug("enable_vq index=%u fd=%d r=%d vq=%p", index, fd, r, vq);
> > > > >                       goto err_used;
> > > > > +             }
> > > > >               if (index == VHOST_NET_VQ_RX)
> > > > >                       nvq->rx_ring = get_tap_ptr_ring(fd);
> > > > > 
> > > > > @@ -1559,6 +1570,8 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
> > > > > 
> > > > >       mutex_unlock(&vq->mutex);
> > > > > 
> > > > > +     pr_debug("sock=%p", sock);
> > > > > +
> > > > >       if (oldubufs) {
> > > > >               vhost_net_ubuf_put_wait_and_free(oldubufs);
> > > > >               mutex_lock(&vq->mutex);
> > > > > @@ -1710,6 +1723,7 @@ static long vhost_net_ioctl(struct file *f, unsigned int ioctl,
> > > > > 
> > > > >       switch (ioctl) {
> > > > >       case VHOST_NET_SET_BACKEND:
> > > > > +             pr_debug("VHOST_NET_SET_BACKEND");
> > > > >               if (copy_from_user(&backend, argp, sizeof backend))
> > > > >                       return -EFAULT;
> > > > >               return vhost_net_set_backend(n, backend.index, backend.fd);
> > > > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > > > index b5a51b1f2e79..ec25ba32fe81 100644
> > > > > --- a/drivers/vhost/vhost.c
> > > > > +++ b/drivers/vhost/vhost.c
> > > > > @@ -1642,15 +1642,30 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
> > > > >                       r = -EINVAL;
> > > > >                       break;
> > > > >               }
> > > > > +
> > > > > +             if (vq->last_avail_idx || vq->avail_idx) {
> > > > > +                     pr_debug(
> > > > > +                             "strange VHOST_SET_VRING_BASE [vq=%p][s.index=%u][s.num=%u]",
> > > > > +                             vq, s.index, s.num);
> > > > > +                     dump_stack();
> > > > > +                     r = 0;
> > > > > +                     break;
> > > > > +             }
> > > > >               vq->last_avail_idx = s.num;
> > > > >               /* Forget the cached index value. */
> > > > >               vq->avail_idx = vq->last_avail_idx;
> > > > > +             pr_debug(
> > > > > +                     "VHOST_SET_VRING_BASE [vq=%p][vq->last_avail_idx=%u][vq-
> > > > > >avail_idx=%u][s.index=%u][s.num=%u]",
> > > > > +                     vq, vq->last_avail_idx, vq->avail_idx, s.index, s.num);
> > > > >               break;
> > > > >       case VHOST_GET_VRING_BASE:
> > > > >               s.index = idx;
> > > > >               s.num = vq->last_avail_idx;
> > > > >               if (copy_to_user(argp, &s, sizeof s))
> > > > >                       r = -EFAULT;
> > > > > +             pr_debug(
> > > > > +                     "VHOST_GET_VRING_BASE [vq=%p][vq->last_avail_idx=%u][vq-
> > > > > >avail_idx=%u][s.index=%u][s.num=%u]",
> > > > > +                     vq, vq->last_avail_idx, vq->avail_idx, s.index, s.num);
> > > > >               break;
> > > > >       case VHOST_SET_VRING_KICK:
> > > > >               if (copy_from_user(&f, argp, sizeof f)) {
> > > > > @@ -2239,8 +2254,8 @@ static int fetch_buf(struct vhost_virtqueue *vq)
> > > > >               vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
> > > > > 
> > > > >               if (unlikely((u16)(vq->avail_idx - last_avail_idx) > vq->num)) {
> > > > > -                     vq_err(vq, "Guest moved used index from %u to %u",
> > > > > -                             last_avail_idx, vq->avail_idx);
> > > > > +                     vq_err(vq, "Guest moved vq %p used index from %u to %u",
> > > > > +                             vq, last_avail_idx, vq->avail_idx);
> > > > >                       return -EFAULT;
> > > > >               }
> > > > > 
> > > > > @@ -2316,6 +2331,9 @@ static int fetch_buf(struct vhost_virtqueue *vq)
> > > > >       BUG_ON(!(vq->used_flags & VRING_USED_F_NO_NOTIFY));
> > > > > 
> > > > >       /* On success, increment avail index. */
> > > > > +     pr_debug(
> > > > > +             "[vq=%p][vq->last_avail_idx=%u][vq->avail_idx=%u][vq->ndescs=%d][vq->first_desc=%d]",
> > > > > +             vq, vq->last_avail_idx, vq->avail_idx, vq->ndescs, vq->first_desc);
> > > > >       vq->last_avail_idx++;
> > > > > 
> > > > >       return 0;
> > > > > @@ -2432,6 +2450,9 @@ EXPORT_SYMBOL_GPL(vhost_get_vq_desc);
> > > > >  /* Reverse the effect of vhost_get_vq_desc. Useful for error handling. */
> > > > >  void vhost_discard_vq_desc(struct vhost_virtqueue *vq, int n)
> > > > >  {
> > > > > +     pr_debug(
> > > > > +             "DISCARD [vq=%p][vq->last_avail_idx=%u][vq->avail_idx=%u][n=%d]",
> > > > > +             vq, vq->last_avail_idx, vq->avail_idx, n);
> > > > >       vq->last_avail_idx -= n;
> > > > >  }
> > > > >  EXPORT_SYMBOL_GPL(vhost_discard_vq_desc);
> > > > > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > > > > index 661088ae6dc7..08f6d2ccb697 100644
> > > > > --- a/drivers/vhost/vhost.h
> > > > > +++ b/drivers/vhost/vhost.h
> > > > > @@ -250,11 +250,11 @@ int vhost_init_device_iotlb(struct vhost_dev *d, bool enabled);
> > > > >       } while (0)
> > > > > 
> > > > >  enum {
> > > > > -     VHOST_FEATURES = (1ULL << VIRTIO_F_NOTIFY_ON_EMPTY) |
> > > > > -                      (1ULL << VIRTIO_RING_F_INDIRECT_DESC) |
> > > > > -                      (1ULL << VIRTIO_RING_F_EVENT_IDX) |
> > > > > -                      (1ULL << VHOST_F_LOG_ALL) |
> > > > > -                      (1ULL << VIRTIO_F_ANY_LAYOUT) |
> > > > > +     VHOST_FEATURES = /* (1ULL << VIRTIO_F_NOTIFY_ON_EMPTY) | */
> > > > > +                      /* (1ULL << VIRTIO_RING_F_INDIRECT_DESC) | */
> > > > > +                      /* (1ULL << VIRTIO_RING_F_EVENT_IDX) | */
> > > > > +                      /* (1ULL << VHOST_F_LOG_ALL) | */
> > > > > +                      /* (1ULL << VIRTIO_F_ANY_LAYOUT) | */
> > > > >                        (1ULL << VIRTIO_F_VERSION_1)
> > > > >  };
> > > > > 
> > > > > 

Can you try the inlined patch over 52c36ce7f334 ("vhost: use batched version by default")? My intention is to check if
"strange VHOST_SET_VRING_BASE" line appears. In previous tests, it appears very fast, but maybe it takes some time for
it to appear, or it does not appear anymore.

Thanks!

From 756a96e489688b2c04230580770aac17c8a46265 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Eugenio=20P=C3=A9rez?= <eperezma@redhat.com>
Date: Fri, 14 Feb 2020 08:02:26 +0100
Subject: [PATCH] vhost: disable all features and trace last_avail_idx and
 ioctl calls

---
 drivers/vhost/net.c   | 20 +++++++++++++++++---
 drivers/vhost/vhost.c | 25 +++++++++++++++++++++++--
 drivers/vhost/vhost.h | 10 +++++-----
 3 files changed, 45 insertions(+), 10 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index e158159671fa..e4d5f843f9c0 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -1505,10 +1505,13 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
 
 	mutex_lock(&n->dev.mutex);
 	r = vhost_dev_check_owner(&n->dev);
-	if (r)
+	if (r) {
+		pr_debug("vhost_dev_check_owner index=%u fd=%d rc r=%d", index, fd, r);
 		goto err;
+	}
 
 	if (index >= VHOST_NET_VQ_MAX) {
+		pr_debug("vhost_dev_check_owner index=%u fd=%d MAX=%d", index, fd, VHOST_NET_VQ_MAX);
 		r = -ENOBUFS;
 		goto err;
 	}
@@ -1518,22 +1521,26 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
 
 	/* Verify that ring has been setup correctly. */
 	if (!vhost_vq_access_ok(vq)) {
+		pr_debug("vhost_net_set_backend index=%u fd=%d !vhost_vq_access_ok", index, fd);
 		r = -EFAULT;
 		goto err_vq;
 	}
 	sock = get_socket(fd);
 	if (IS_ERR(sock)) {
 		r = PTR_ERR(sock);
+		pr_debug("vhost_net_set_backend index=%u fd=%d get_socket err r=%d", index, fd, r);
 		goto err_vq;
 	}
 
 	/* start polling new socket */
 	oldsock = vq->private_data;
 	if (sock != oldsock) {
+		pr_debug("sock=%p != oldsock=%p index=%u fd=%d vq=%p", sock, oldsock, index, fd, vq);
 		ubufs = vhost_net_ubuf_alloc(vq,
 					     sock && vhost_sock_zcopy(sock));
 		if (IS_ERR(ubufs)) {
 			r = PTR_ERR(ubufs);
+			pr_debug("ubufs index=%u fd=%d err r=%d vq=%p", index, fd, r, vq);
 			goto err_ubufs;
 		}
 
@@ -1541,11 +1548,15 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
 		vq->private_data = sock;
 		vhost_net_buf_unproduce(nvq);
 		r = vhost_vq_init_access(vq);
-		if (r)
+		if (r) {
+			pr_debug("init_access index=%u fd=%d r=%d vq=%p", index, fd, r, vq);
 			goto err_used;
+		}
 		r = vhost_net_enable_vq(n, vq);
-		if (r)
+		if (r) {
+			pr_debug("enable_vq index=%u fd=%d r=%d vq=%p", index, fd, r, vq);
 			goto err_used;
+		}
 		if (index == VHOST_NET_VQ_RX)
 			nvq->rx_ring = get_tap_ptr_ring(fd);
 
@@ -1559,6 +1570,8 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
 
 	mutex_unlock(&vq->mutex);
 
+	pr_debug("sock=%p", sock);
+
 	if (oldubufs) {
 		vhost_net_ubuf_put_wait_and_free(oldubufs);
 		mutex_lock(&vq->mutex);
@@ -1710,6 +1723,7 @@ static long vhost_net_ioctl(struct file *f, unsigned int ioctl,
 
 	switch (ioctl) {
 	case VHOST_NET_SET_BACKEND:
+		pr_debug("VHOST_NET_SET_BACKEND");
 		if (copy_from_user(&backend, argp, sizeof backend))
 			return -EFAULT;
 		return vhost_net_set_backend(n, backend.index, backend.fd);
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 56c5253056ee..babc48c8a8c4 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -1640,15 +1640,30 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
 			r = -EINVAL;
 			break;
 		}
+
+		if (vq->last_avail_idx || vq->avail_idx) {
+			pr_debug(
+				"strange VHOST_SET_VRING_BASE [vq=%p][s.index=%u][s.num=%u]",
+				vq, s.index, s.num);
+			dump_stack();
+			r = 0;
+			break;
+		}
 		vq->last_avail_idx = s.num;
 		/* Forget the cached index value. */
 		vq->avail_idx = vq->last_avail_idx;
+		pr_debug(
+			"VHOST_SET_VRING_BASE [vq=%p][vq->last_avail_idx=%u][vq->avail_idx=%u][s.index=%u][s.num=%u]",
+			vq, vq->last_avail_idx, vq->avail_idx, s.index, s.num);
 		break;
 	case VHOST_GET_VRING_BASE:
 		s.index = idx;
 		s.num = vq->last_avail_idx;
 		if (copy_to_user(argp, &s, sizeof s))
 			r = -EFAULT;
+		pr_debug(
+			"VHOST_GET_VRING_BASE [vq=%p][vq->last_avail_idx=%u][vq->avail_idx=%u][s.index=%u][s.num=%u]",
+			vq, vq->last_avail_idx, vq->avail_idx, s.index, s.num);
 		break;
 	case VHOST_SET_VRING_KICK:
 		if (copy_from_user(&f, argp, sizeof f)) {
@@ -2233,8 +2248,8 @@ static int fetch_descs(struct vhost_virtqueue *vq)
 		vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
 
 		if (unlikely((u16)(vq->avail_idx - last_avail_idx) > vq->num)) {
-			vq_err(vq, "Guest moved used index from %u to %u",
-				last_avail_idx, vq->avail_idx);
+			vq_err(vq, "Guest moved vq %p used index from %u to %u",
+				vq, last_avail_idx, vq->avail_idx);
 			return -EFAULT;
 		}
 
@@ -2310,6 +2325,9 @@ static int fetch_descs(struct vhost_virtqueue *vq)
 	BUG_ON(!(vq->used_flags & VRING_USED_F_NO_NOTIFY));
 
 	/* On success, increment avail index. */
+	pr_debug(
+		"[vq=%p][vq->last_avail_idx=%u][vq->avail_idx=%u][vq->ndescs=%d][vq->first_desc=%d]",
+		vq, vq->last_avail_idx, vq->avail_idx, vq->ndescs, vq->first_desc);
 	vq->last_avail_idx++;
 
 	return 0;
@@ -2403,6 +2421,9 @@ EXPORT_SYMBOL_GPL(vhost_get_vq_desc);
 /* Reverse the effect of vhost_get_vq_desc. Useful for error handling. */
 void vhost_discard_vq_desc(struct vhost_virtqueue *vq, int n)
 {
+	pr_debug(
+		"DISCARD [vq=%p][vq->last_avail_idx=%u][vq->avail_idx=%u][n=%d]",
+		vq, vq->last_avail_idx, vq->avail_idx, n);
 	vq->last_avail_idx -= n;
 }
 EXPORT_SYMBOL_GPL(vhost_discard_vq_desc);
diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index a0bcf8bffa43..2ce2d3a97c31 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -248,11 +248,11 @@ int vhost_init_device_iotlb(struct vhost_dev *d, bool enabled);
 	} while (0)
 
 enum {
-	VHOST_FEATURES = (1ULL << VIRTIO_F_NOTIFY_ON_EMPTY) |
-			 (1ULL << VIRTIO_RING_F_INDIRECT_DESC) |
-			 (1ULL << VIRTIO_RING_F_EVENT_IDX) |
-			 (1ULL << VHOST_F_LOG_ALL) |
-			 (1ULL << VIRTIO_F_ANY_LAYOUT) |
+	VHOST_FEATURES = /* (1ULL << VIRTIO_F_NOTIFY_ON_EMPTY) | */
+			 /* (1ULL << VIRTIO_RING_F_INDIRECT_DESC) | */
+			 /* (1ULL << VIRTIO_RING_F_EVENT_IDX) | */
+			 /* (1ULL << VHOST_F_LOG_ALL) | */
+			 /* (1ULL << VIRTIO_F_ANY_LAYOUT) | */
 			 (1ULL << VIRTIO_F_VERSION_1)
 };
 
-- 
2.18.1