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: Thu, 6 Feb 2020 17:07:20 -0500

On Thu, Feb 06, 2020 at 03:22:39PM +0100, [email-masked] wrote:
> Hi Christian.
> 
> Could you try this patch on top of ("38ced0208491 vhost: use batched version by default")?
> 
> It will not solve your first random crash but it should help with the lost of network connectivity.
> 
> Please let me know how does it goes.
> 
> Thanks!
> 
> >From 99f0f543f3939dbe803988c9153a95616ccccacd Mon Sep 17 00:00:00 2001
> From: =?UTF-8?q?Eugenio=20P=C3=A9rez?= <[email-masked]>
> Date: Thu, 6 Feb 2020 15:13:42 +0100
> Subject: [PATCH] vhost: filter valid vhost descriptors flags
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
> 
> Previous commit copy _NEXT flag, and it complains if a copied descriptor
> contains it.
> 
> Signed-off-by: Eugenio Pérez <[email-masked]>
> ---
>  drivers/vhost/vhost.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 27ae5b4872a0..56c5253056ee 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2125,6 +2125,8 @@ static void pop_split_desc(struct vhost_virtqueue *vq)
>  	--vq->ndescs;
>  }
>  
> +#define VHOST_DESC_FLAGS (VRING_DESC_F_INDIRECT | VRING_DESC_F_WRITE | \
> +			  VRING_DESC_F_NEXT)
>  static int push_split_desc(struct vhost_virtqueue *vq, struct vring_desc *desc, u16 id)
>  {
>  	struct vhost_desc *h;
> @@ -2134,7 +2136,7 @@ static int push_split_desc(struct vhost_virtqueue *vq, struct vring_desc *desc,
>  	h = &vq->descs[vq->ndescs++];
>  	h->addr = vhost64_to_cpu(vq, desc->addr);
>  	h->len = vhost32_to_cpu(vq, desc->len);
> -	h->flags = vhost16_to_cpu(vq, desc->flags);
> +	h->flags = vhost16_to_cpu(vq, desc->flags) & VHOST_DESC_FLAGS;
>  	h->id = id;
>  
>  	return 0;

> @@ -2343,7 +2345,7 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
>  		struct vhost_desc *desc = &vq->descs[i];
>  		int access;
>  
> -		if (desc->flags & ~(VRING_DESC_F_INDIRECT | VRING_DESC_F_WRITE)) {
> +		if (desc->flags & ~VHOST_DESC_FLAGS) {
>  			vq_err(vq, "Unexpected flags: 0x%x at descriptor id 0x%x\n",
>  			       desc->flags, desc->id);
>  			ret = -EINVAL;
> -- 
> 2.18.1

Thanks for catching this!

Do we need the 1st chunk though?

It seems preferable to just muck with flags in 1 place, when we
validate them ...

> 
> On Wed, 2020-01-22 at 20:32 +0100, Christian Borntraeger wrote:
> > 
> > On 20.01.20 07:27, Michael S. Tsirkin wrote:
> > > On Tue, Jan 07, 2020 at 01:16:50PM +0100, Christian Borntraeger wrote:
> > > > On 07.01.20 12:55, Michael S. Tsirkin wrote:
> > > > 
> > > > > I pushed batched-v3 - same head but bisect should work now.
> > > > > 
> > > > 
> > > > With 
> > > > commit 38ced0208491103b50f1056f0d1c8f28e2e13d08 (HEAD)
> > > > Author:     Michael S. Tsirkin <[email-masked]>
> > > > AuthorDate: Wed Dec 11 12:19:26 2019 -0500
> > > > Commit:     Michael S. Tsirkin <[email-masked]>
> > > > CommitDate: Tue Jan 7 06:52:42 2020 -0500
> > > > 
> > > >     vhost: use batched version by default
> > > > 
> > > > 
> > > > I have exactly one successful ping and then the network inside the guest is broken (no packet
> > > > anymore).
> > > 
> > > Does anything appear in host's dmesg when this happens?
> > 
> > I think there was nothing, but I am not sure. I would need to redo the test if this is important to know.
> > 
> > > 
> > > > So you could consider this commit broken (but in a different way and also without any
> > > > guest reboot necessary).
> > > > 
> > > > 
> > > > bisect log:
> > > > git bisect start
> > > > # bad: [d2f6175f52062ee51ee69754a6925608213475d2] vhost: use vhost_desc instead of vhost_log
> > > > git bisect bad d2f6175f52062ee51ee69754a6925608213475d2
> > > > # good: [d1281e3a562ec6a08f944a876481dd043ba739b9] virtio-blk: remove VIRTIO_BLK_F_SCSI support
> > > > git bisect good d1281e3a562ec6a08f944a876481dd043ba739b9
> > > > # good: [fac7c0f46996e32d996f5c46121df24a6b95ec3b] vhost: option to fetch descriptors through an independent
> > > > struct
> > > > git bisect good fac7c0f46996e32d996f5c46121df24a6b95ec3b
> > > > # bad: [539eb9d738f048cd7be61f404e8f9c7d9d2ff3cc] vhost: batching fetches
> > > > git bisect bad 539eb9d738f048cd7be61f404e8f9c7d9d2ff3cc