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

[Stefano Garzarella <sgarzare@redhat.com>]

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@vger.kernel.org
>> > > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
>> > > ---
>> > >
>> > > 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@amd.com/
>> >
>> > 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