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Date:   Wed, 7 Jun 2023 10:39:15 +0200
From:   Stefano Garzarella <sgarzare@redhat.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Jason Wang <jasowang@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 6, 2023 at 2:58 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> 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
> > 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
>
>
> I have a question though, what if down the road there
> is a new feature that needs more changes? It will be
> broken too just like PACKED no?
> Shouldn't vdpa have an allowlist of features it knows how
> to support?

It looks like we had it, but we took it out (by the way, we were
enabling packed even though we didn't support it):
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6234f80574d7569444d8718355fa2838e92b158b

The only problem I see is that for each new feature we have to modify 
the kernel.
Could we have new features that don't require handling by vhost-vdpa?

Thanks,
Stefano