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

From: Eugenio Pérez <[email-masked]>
Date: Thu, 13 Feb 2020 11:47:23 +0100

On Thu, 2020-02-13 at 10:30 +0100, Christian Borntraeger wrote:
> 
> On 12.02.20 17:34, Eugenio Pérez wrote:
> > On Tue, 2020-02-11 at 14:13 +0100, Christian Borntraeger wrote:
> > > On 11.02.20 14:04, Eugenio Pérez wrote:
> > > > On Mon, 2020-02-10 at 12:01 +0100, Christian Borntraeger wrote:
> > > > > On 10.02.20 10:47, Eugenio Perez Martin wrote:
> > > > > > Hi Christian.
> > > > > > 
> > > > > > I'm not able to reproduce the failure with eccb852f1fe6bede630e2e4f1a121a81e34354ab commit. Could you add
> > > > > > more
> > > > > > data?
> > > > > > Your configuration (libvirt or qemu line), and host's dmesg output if any?
> > > > > > 
> > > > > > Thanks!
> > > > > 
> > > > > If it was not obvious, this is on s390x, a big endian system.
> > > > > 
> > > > 
> > > > Hi Christian. Thank you very much for your fast responses.
> > > > 
> > > > Could you try this patch on top of eccb852f1fe6bede630e2e4f1a121a81e34354ab?
> > > 
> > > I still get 
> > > [   43.665145] Guest moved used index from 0 to 289
> > > after some reboots.
> > > 
> > > 
> > > > Thanks!
> > > > 
> > > > From 71d0f9108a18aa894cc0c0c1c7efbad39f465a27 Mon Sep 17 00:00:00 2001
> > > > From: =?UTF-8?q?Eugenio=20P=C3=A9rez?= <
> > > > [email-masked]>
> > > > Date: Tue, 11 Feb 2020 13:19:10 +0100
> > > > Subject: [PATCH] vhost: fix return value of vhost_get_vq_desc
> > > > 
> > > > Before of the batch change, it was the chain's head. Need to keep that
> > > > way or we will not be able to free a chain of descriptors.
> > > > 
> > > > Fixes: eccb852f1fe6 ("vhost: batching fetches")
> > > > ---
> > > >  drivers/vhost/vhost.c | 3 +--
> > > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > > > 
> > > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > > index b5a51b1f2e79..fc422c3e5c08 100644
> > > > --- a/drivers/vhost/vhost.c
> > > > +++ b/drivers/vhost/vhost.c
> > > > @@ -2409,12 +2409,11 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> > > >  			*out_num += ret;
> > > >  		}
> > > >  
> > > > -		ret = desc->id;
> > > > -
> > > >  		if (!(desc->flags & VRING_DESC_F_NEXT))
> > > >  			break;
> > > >  	}
> > > >  
> > > > +	ret = vq->descs[vq->first_desc].id;
> > > >  	vq->first_desc = i + 1;
> > > >  
> > > >  	return ret;
> > > > 
> > 
> > Sorry, still not able to reproduce the issue.
> > 
> > Could we try to disable all the vhost features?
> > 
> > Thanks!
> > 
> > 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)
> >  };
> > 
> 
> I still get  guest crashes with this on top of eccb852f1fe6. (The patch did not
> apply, I had to manually comment out these things)
> 

Sorry about that, I C&P transformed tabs to spaces.

Can we try tracing last_avail_idx with the attached patch? Can you enable also line and thread id (dyndbg='+plt')?

Thanks!

>From f7012e8b9db711b12d36e6e97411e7afa34bf768 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Eugenio=20P=C3=A9rez?= <[email-masked]>
Date: Thu, 13 Feb 2020 11:26:06 +0100
Subject: [PATCH] vhost: disable all features and trace last_avail_idx

---
 drivers/vhost/vhost.c | 13 +++++++++++--
 drivers/vhost/vhost.h | 10 +++++-----
 2 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index fc422c3e5c08..021d70bed015 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -1645,6 +1645,9 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
 		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]",
+			vq, vq->last_avail_idx, vq->avail_idx);
 		break;
 	case VHOST_GET_VRING_BASE:
 		s.index = idx;
@@ -2239,8 +2242,8 @@ 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 +2319,9 @@ 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;
@@ -2431,6 +2437,9 @@ 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)
 };
 
-- 
2.18.1