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From:   Jason Wang <jasowang@redhat.com>
Date:   Tue, 6 Jun 2023 09:29:22 +0800
Message-ID: <CACGkMEu3PqQ99UoKF5NHgVADD3q=BF6jhLiyumeT4S1QCqN1tw@mail.gmail.com>
Subject: Re: [PATCH] vhost-vdpa: filter VIRTIO_F_RING_PACKED feature
To:     Stefano Garzarella <sgarzare@redhat.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        Shannon Nelson <shannon.nelson@amd.com>,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org,
        =?UTF-8?Q?Eugenio_P=C3=A9rez?= <eperezma@redhat.com>,
        Tiwei Bie <tiwei.bie@intel.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
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On Mon, Jun 5, 2023 at 10:58=E2=80=AFPM 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 wro=
te:
> >> > > > > vhost-vdpa IOCTLs (eg. VHOST_GET_VRING_BASE, VHOST_SET_VRING_B=
ASE)
> >> > > > > don't support packed virtqueue well yet, so let's filter the
> >> > > > > VIRTIO_F_RING_PACKED feature for now in vhost_vdpa_get_feature=
s().
> >> > > > >
> >> > > > > This way, even if the device supports it, we don't risk it bei=
ng
> >> > > > > negotiated, then the VMM is unable to set the vring state prop=
erly.
> >> > > > >
> >> > > > > 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] vh=
ost_vdpa:
> >> > > > >     better PACKED support" series [1] and backported in stable=
 branches.
> >> > > > >
> >> > > > >     We can revert it when we are sure that everything is worki=
ng 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 win=
dow.
> >> > > 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 rever=
t this
> >> > > patch.
> >> > >
> >> > > Let me know if you prefer a different approach.
> >> > >
> >> > > I'm concerned that QEMU uses vhost-vdpa IOCTLs thinking that the k=
ernel
> >> > > 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
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.

Thanks

>
> Thanks,
> Stefano
>
>