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Date:   Wed, 13 Oct 2021 08:24:53 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Christian Borntraeger <borntraeger@de.ibm.com>
Cc:     Halil Pasic <pasic@linux.ibm.com>,
        Jason Wang <jasowang@redhat.com>,
        Xie Yongji <xieyongji@bytedance.com>,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        markver@us.ibm.com, Cornelia Huck <cohuck@redhat.com>,
        linux-s390@vger.kernel.org, stefanha@redhat.com,
        Raphael Norwitz <raphael.norwitz@nutanix.com>,
        qemu-devel@nongnu.org
Subject: Re: [PATCH v3 1/1] virtio: write back F_VERSION_1 before validate
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On Wed, Oct 13, 2021 at 01:23:50PM +0200, Christian Borntraeger wrote:
> 
> 
> 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?

Halil said he'll do it... Right, Halil?

> 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?

Probably. Still trying to decide between this and plain revert for 5.15
and back. Maybe both?

> > 
> > > ---
> > > 
> > > @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
> >