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Subject: Re: vhost changes (batched) in linux-next after 12/13 trigger
 random crashes in KVM guests after reboot
From:   Eugenio =?ISO-8859-1?Q?P=E9rez?= <eperezma@redhat.com>
To:     Christian Borntraeger <borntraeger@de.ibm.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        "virtualization@lists.linux-foundation.org" 
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        Cornelia Huck <cohuck@redhat.com>
Date:   Wed, 12 Feb 2020 17:34:16 +0100
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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?= <
> > eperezma@redhat.com>
> > 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)
 };