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From:   Jason Wang <jasowang@redhat.com>
Date:   Thu, 8 Jun 2023 15:46:00 +0800
Message-ID: <CACGkMEt4=3BRVNX38AD+mJU8v3bmqO-CdNj5NkFP-SSvsuy2Hg@mail.gmail.com>
Subject: Re: [PATCH] vhost-vdpa: filter VIRTIO_F_RING_PACKED feature
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Stefano Garzarella <sgarzare@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 Thu, Jun 8, 2023 at 2:03=E2=80=AFPM Michael S. Tsirkin <mst@redhat.com> =
wrote:
>
> On Thu, Jun 08, 2023 at 08:42:15AM +0800, Jason Wang wrote:
> > On Wed, Jun 7, 2023 at 5:43=E2=80=AFPM Michael S. Tsirkin <mst@redhat.c=
om> wrote:
> > >
> > > On Wed, Jun 07, 2023 at 10:39:15AM +0200, Stefano Garzarella wrote:
> > > > On Tue, Jun 6, 2023 at 2:58=E2=80=AFPM Michael S. Tsirkin <mst@redh=
at.com> wrote:
> > > > >
> > > > > On Tue, Jun 06, 2023 at 09:29:22AM +0800, Jason Wang wrote:
> > > > > > On Mon, Jun 5, 2023 at 10:58=E2=80=AFPM Stefano Garzarella <sga=
rzare@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. Tsirk=
in wrote:
> > > > > > > >> > On Mon, Jun 05, 2023 at 02:54:20PM +0200, Stefano Garzar=
ella wrote:
> > > > > > > >> > > On Mon, Jun 05, 2023 at 08:41:54AM -0400, Michael S. T=
sirkin wrote:
> > > > > > > >> > > > On Mon, Jun 05, 2023 at 01:06:44PM +0200, Stefano Ga=
rzarella 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 vrin=
g 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 "[PATC=
H v2 0/3] vhost_vdpa:
> > > > > > > >> > > > >     better PACKED support" series [1] and backport=
ed in stable branches.
> > > > > > > >> > > > >
> > > > > > > >> > > > >     We can revert it when we are sure that everyth=
ing is working with
> > > > > > > >> > > > >     packed virtqueues.
> > > > > > > >> > > > >
> > > > > > > >> > > > >     Thanks,
> > > > > > > >> > > > >     Stefano
> > > > > > > >> > > > >
> > > > > > > >> > > > >     [1] https://lore.kernel.org/virtualization/202=
30424225031.18947-1-shannon.nelson@amd.com/
> > > > > > > >> > > >
> > > > > > > >> > > > I'm a bit lost here. So why am I merging "better PAC=
KED 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 th=
is merge window.
> > > > > > > >> > > In that case do you think it is better to send this pa=
tch 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 thinkin=
g 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 th=
em? Then it's ok
> > > > > > > >> > to merge in this window. Probably easier than the elabor=
ate
> > > > > > > >> > mask/unmask dance.
> > > > > > > >>
> > > > > > > >> CCing Shannon (the original author of the "better PACKED s=
upport"
> > > > > > > >> series).
> > > > > > > >>
> > > > > > > >> IIUC Shannon is going to send a v3 of that series to fix t=
he
> > > > > > > >> 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 pa=
cked
> > > > > > > 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 thinki=
ng of
> > > > > > >
> > > > > > > Fixes: 530a5678bc00 ("vdpa: support packed virtqueue for set/=
get_vq_state()")
> > > > > > >
> > > > > > > So we would have to backport quite a few patches into the sta=
ble branches.
> > > > > > > I don't know if it's worth it...
> > > > > > >
> > > > > > > I still think it is better to disable packed in the stable br=
anches,
> > > > > > > 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 tha=
t
> > > > > > supports packed virtqueue. Users should not notice anything wro=
ng if
> > > > > > they don't use packed virtqueue. And the problem of vp_vdpa + p=
acked
> > > > > > virtqueue came since the day0 of vp_vdpa. It seems fine to do n=
othing
> > > > > > 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=3D6234f80574d7569444d8718355fa2838e92b158b
> > > >
> > > > The only problem I see is that for each new feature we have to modi=
fy
> > > > the kernel.
> > > > Could we have new features that don't require handling by vhost-vdp=
a?
> > > >
> > > > Thanks,
> > > > Stefano
> > >
> > > Jason what do you say to reverting this?
> >
> > I may miss something but I don't see any problem with vDPA core.
> >
> > It's the duty of the parents to advertise the features it has. For exam=
ple,
> >
> > 1) If some kernel version that is packed is not supported via
> > set_vq_state, parents should not advertise PACKED features in this
> > case.
> > 2) If the kernel has support packed set_vq_state(), but it's emulated
> > cvq doesn't support, parents should not advertise PACKED as well
> >
> > If a parent violates the above 2, it looks like a bug of the parents.
> >
> > Thanks
>
> Yes but what about vhost_vdpa? Talking about that not the core.

Not sure it's a good idea to workaround parent bugs via vhost-vDPA.

> Should that not have a whitelist of features
> since it interprets ioctls differently depending on this?

If there's a bug, it might only matter the following setup:

SET_VRING_BASE/GET_VRING_BASE + VDUSE.

This seems to be broken since VDUSE was introduced. If we really want
to backport something, it could be a fix to filter out PACKED in
VDUSE?

Thanks

>
> > >
> > > --
> > > MST
> > >
>