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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 11 Feb 2020 08:57:58 -0500

On Tue, Feb 11, 2020 at 02:04:54PM +0100, 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?
> 
> 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.

I think it's cleaner to have all descriptors in the chain
have the same id.

> 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;
> -- 
> 2.18.1