On Fri, Oct 08, 2021 at 03:51:56PM +0200, Halil Pasic wrote:
> On Fri, 8 Oct 2021 09:05:03 -0400
> "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
>
> > On Fri, Oct 08, 2021 at 02:34:22PM +0200, Halil Pasic wrote:
> > > The virtio specification virtio-v1.1-cs01 states: "Transitional devices
> > > MUST detect Legacy drivers by detecting that VIRTIO_F_VERSION_1 has not
> > > been acknowledged by the driver." This is exactly what QEMU as of 6.1
> > > has done relying solely on VIRTIO_F_VERSION_1 for detecting that.
> > >
> > > However, the specification also says: "... the driver MAY read (but MUST
> > > NOT write) the device-specific configuration fields to check that it can
> > > support the device ..." before setting FEATURES_OK.
> > >
> > > In that case, any transitional device relying solely on
> > > VIRTIO_F_VERSION_1 for detecting legacy drivers will return data in
> > > legacy format. In particular, this implies that it is in big endian
> > > format for big endian guests. This naturally confuses the driver which
> > > expects little endian in the modern mode.
> > >
> > > It is probably a good idea to amend the spec to clarify that
> > > VIRTIO_F_VERSION_1 can only be relied on after the feature negotiation
> > > is complete. However, we already have a regression so let's try to address
> >
> > actually, regressions. and we can add
> > "since originally before validate callback existed
> > config space was only read after
> > FEATURES_OK. See Fixes tags for relevant commits"
> >
> > > it.
>
> How about replacing the paragraph above with the following?
>
> "It is probably a good idea to amend the spec to clarify that
> VIRTIO_F_VERSION_1 can only be relied on after the feature negotiation
> is complete. Before validate callback existed, config space was only
> read after FEATURES_OK. However, we already have two regression,
two regressions
> so
> let's address this here as well."
> > >
> > > The regressions affect the VIRTIO_NET_F_MTU feature of virtio-net and
> > > the VIRTIO_BLK_F_BLK_SIZE feature of virtio-blk for BE guests when
> > > virtio 1.0 is used on both sides. The latter renders virtio-blk
> > > unusable with DASD backing, because things simply don't work with
> > > the default.
>
> and add
> "See Fixes tags for relevant commits."
> here.
> >
> > Let's add a work around description now:
> >
> >
> > For QEMU, we can work around the issue by writing out the features
> > register with VIRTIO_F_VERSION_1 bit set. We (ab) use the
> s/features register/feature bits/
> rationale: ccw does not have a features register, and qemu does not
> really act as if its behavior was controlled by the values in a features
> register. I.e. when we read the register we see VIRTIO_F_VERSION_!
> because the feature is offered. In QEMU we basically read host_featues
> but write the guest_features. And what drives device behavior is mostly
> guest_features.
>
> s/(ab) use/(ab)use/
>
> > finalize_features config op for this. It's not enough to address vhost
>
> s/It's/This is/
>
> > user and vhost block devices since these do not get the features until
>
> s/vhost user and vhost block/some vhost-user and vhost-vdpa/ ?
Let's just say "not enough to address vhost devices since some
of these etc"
> Ratioale: I think vhost block is just a vhost-user device. On the other
> hand vhost-user-fs works like charm because the config space is
> implemented in qemu and not in the vhost-user device. I
> didn't check vhost_net. I'm not even sure qemu offers a vhost_net
> implementation.
it does
> Anyway I wouldn't like to make any false statements here.
ok
> > FEATURES_OK, however it looks like these two actually never handled the
> > endian-ness for legacy mode correctly, so at least that's not a
> > regression.
> >
> > No devices except virtio net and virtio blk seem to be affected.
> >
> > Long term the right thing to do is to fix the hypervisors.
> >
>
> Sounds good. Thanks! Are you OK with my changes proposed to your changes?
>
> Regards,
> Halil
yes.
> >
> > >
> > > Cc: <stable@xxxxxxxxxxxxxxx> #v4.11
> > > Signed-off-by: Halil Pasic <pasic@xxxxxxxxxxxxx>
> > > Fixes: 82e89ea077b9 ("virtio-blk: Add validation for block size in
> > > config space") Fixes: fe36cbe0671e ("virtio_net: clear MTU when out
> > > of range") Reported-by: markver@xxxxxxxxxx
> > > ---
> > > drivers/virtio/virtio.c | 11 +++++++++++
> > > 1 file changed, 11 insertions(+)
> > >
> > > diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> > > index 0a5b54034d4b..236081afe9a2 100644
> > > --- a/drivers/virtio/virtio.c
> > > +++ b/drivers/virtio/virtio.c
> > > @@ -239,6 +239,17 @@ static int virtio_dev_probe(struct device *_d)
> > > driver_features_legacy = driver_features;
> > > }
> > >
> > > + /*
> > > + * Some devices detect legacy solely via F_VERSION_1. Write
> > > + * F_VERSION_1 to force LE config space accesses before
> > > FEATURES_OK for
> > > + * these when needed.
> > > + */
> > > + if (drv->validate && !virtio_legacy_is_little_endian()
> > > + && device_features &
> > > BIT_ULL(VIRTIO_F_VERSION_1)) {
> > > + dev->features = BIT_ULL(VIRTIO_F_VERSION_1);
> > > + dev->config->finalize_features(dev);
> > > + }
> > > +
> > > if (device_features & (1ULL << VIRTIO_F_VERSION_1))
> > > dev->features = driver_features & device_features;
> > > else
> > >
> > > base-commit: 60a9483534ed0d99090a2ee1d4bb0b8179195f51
> > > --
> > > 2.25.1
> >
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