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Date:   Tue, 6 Jun 2023 12:18:40 +0200
From:   Stefano Garzarella <sgarzare@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        Shannon Nelson <shannon.nelson@amd.com>,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org,
        Eugenio =?utf-8?B?UMOpcmV6?= <eperezma@redhat.com>,
        Tiwei Bie <tiwei.bie@intel.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] vhost-vdpa: filter VIRTIO_F_RING_PACKED feature
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On Tue, Jun 06, 2023 at 09:29:22AM +0800, Jason Wang wrote:
>On Mon, Jun 5, 2023 at 10:58 PM Stefano Garzarella <sgarzare@redhat.com> wrote:
>>
>> 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@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
>> >
>> >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?
>
>AFAIK, except for vp_vdpa, pds seems to be the first parent that

IIUC also vduse and snet supports packed virtqueue.

>supports packed virtqueue. Users should not notice anything wrong if
>they don't use packed virtqueue. And the problem of vp_vdpa + packed
>virtqueue came since the day0 of vp_vdpa. It seems fine to do nothing
>I guess.

Okay, maybe I'm overthinking it, not having a specific problem I don't
object, it was just a concern about uAPI.

Thanks,
Stefano