Re: [PATCH] vhost-vdpa: filter VIRTIO_F_RING_PACKED feature

[Stefano Garzarella]

On Mon, Jun 05, 2023 at 08:41:54AM -0400, Michael S. Tsirkin wrote:
> On Mon, Jun 05, 2023 at 01:06:44PM +0200, Stefano Garzarella wrote:
> > vhost-vdpa IOCTLs (eg. VHOST_GET_VRING_BASE, VHOST_SET_VRING_BASE)
> > don't support packed virtqueue well yet, so let's filter the
> > VIRTIO_F_RING_PACKED feature for now in vhost_vdpa_get_features().
> >
> > This way, even if the device supports it, we don't risk it being
> > negotiated, then the VMM is unable to set the vring state properly.
> >
> > Fixes: 4c8cf31885f6 ("vhost: introduce vDPA-based backend")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> > ---
> >
> > Notes:
> >     This patch should be applied before the "[PATCH v2 0/3] vhost_vdpa:
> >     better PACKED support" series [1] and backported in stable branches.
> >
> >     We can revert it when we are sure that everything is working with
> >     packed virtqueues.
> >
> >     Thanks,
> >     Stefano
> >
> >     [1] https://lore.kernel.org/virtualization/20230424225031.18947-1-shannon.nelson@xxxxxxx/
>
> I'm a bit lost here. So why am I merging "better PACKED support" then?

To really support packed virtqueue with vhost-vdpa, at that point we 
would also have to revert this patch.

I wasn't sure if you wanted to queue the series for this merge window.
In that case do you think it is better to send this patch only for 
stable branches?

> Does this patch make them a NOP?

Yep, after applying the "better PACKED support" series and being sure 
that the IOCTLs of vhost-vdpa support packed virtqueue, we should revert 
this patch.

Let me know if you prefer a different approach.

I'm concerned that QEMU uses vhost-vdpa IOCTLs thinking that the kernel 
interprets them the right way, when it does not.

Thanks,
Stefano

> >  drivers/vhost/vdpa.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> > index 8c1aefc865f0..ac2152135b23 100644
> > --- a/drivers/vhost/vdpa.c
> > +++ b/drivers/vhost/vdpa.c
> > @@ -397,6 +397,12 @@ static long vhost_vdpa_get_features(struct vhost_vdpa *v, u64 __user *featurep)
> >
> >  	features = ops->get_device_features(vdpa);
> >
> > +	/*
> > +	 * IOCTLs (eg. VHOST_GET_VRING_BASE, VHOST_SET_VRING_BASE) don't support
> > +	 * packed virtqueue well yet, so let's filter the feature for now.
> > +	 */
> > +	features &= ~BIT_ULL(VIRTIO_F_RING_PACKED);
> > +
> >  	if (copy_to_user(featurep, &features, sizeof(features)))
> >  		return -EFAULT;
> >
> >
> > base-commit: 9561de3a55bed6bdd44a12820ba81ec416e705a7
> > --
> > 2.40.1