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

[Stefano Garzarella]

On Mon, Jun 05, 2023 at 09:54:57AM -0400, Michael S. Tsirkin wrote:
> On Mon, Jun 05, 2023 at 03:30:35PM +0200, Stefano Garzarella wrote:
> > On Mon, Jun 05, 2023 at 09:00:25AM -0400, Michael S. Tsirkin wrote:
> > > On Mon, Jun 05, 2023 at 02:54:20PM +0200, Stefano Garzarella wrote:
> > > > 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
> > > >
> > >
> > > If this fixes a bug can you add Fixes tags to each of them? Then it's ok
> > > to merge in this window. Probably easier than the elaborate
> > > mask/unmask dance.
> >
> > CCing Shannon (the original author of the "better PACKED support"
> > series).
> >
> > IIUC Shannon is going to send a v3 of that series to fix the
> > documentation, so Shannon can you also add the Fixes tags?
> >
> > Thanks,
> > Stefano
>
> Well this is in my tree already. Just reply with
> Fixes: <>
> to each and I will add these tags.

I tried, but it is not easy since we added the support for packed 
virtqueue in vdpa and vhost incrementally.

Initially I was thinking of adding the same tag used here:

Fixes: 4c8cf31885f6 ("vhost: introduce vDPA-based backend")

Then I discovered that vq_state wasn't there, so I was thinking of

Fixes: 530a5678bc00 ("vdpa: support packed virtqueue for set/get_vq_state()")

So we would have to backport quite a few patches into the stable branches.
I don't know if it's worth it...

I still think it is better to disable packed in the stable branches,
otherwise I have to make a list of all the patches we need.

Any other ideas?

Thanks,
Stefano