Re: [PATCH v3 1/1] virtio: write back F_VERSION_1 before validate

[Christian Borntraeger]

Am 13.10.21 um 12:10 schrieb Michael S. Tsirkin:
> On Mon, Oct 11, 2021 at 07:39:21AM +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. Before validate callback existed, config space was only
> > read after FEATURES_OK. However, we already have 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.
> > See Fixes tags for relevant commits.
> >
> > For QEMU, we can work around the issue by writing out the feature bits
> > with VIRTIO_F_VERSION_1 bit set.  We (ab)use the finalize_features
> > config op for this. This isn't enough to address all vhost devices since
> > these do not get the features until FEATURES_OK, however it looks like
> > the affected devices actually never handled the endianness 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.
> >
> > Cc: <stable@vger.kernel.org> #v4.11
> > Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
> > 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@us.ibm.com
> > Reviewed-by: Cornelia Huck <cohuck@redhat.com>
>
> OK this looks good! How about a QEMU patch to make it spec compliant on
> BE?

Who is going to do that? Halil? you? Conny?

Can we get this kernel patch queued for 5.15 and stable without waiting for the 
QEMU patch
as we have a regression with 4.14?
> > ---
> >
> > @Connie: I made some more commit message changes to accommodate Michael's
> > requests. I just assumed these will work or you as well and kept your
> > r-b. Please shout at me if it needs to be dropped :)
> > ---
> >   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