| Message ID | 20211109124702.2369200-2-frank.heimes@canonical.com |
|---|---|
| State | New |
| Headers | show |
| Series | kernel: unable to read partitions on virtio-block dasd (kvm) (LP: 1950144) | expand |
On 09/11/2021 13:47, frank.heimes@canonical.com wrote: > From: Halil Pasic <pasic@linux.ibm.com> > > BugLink: https://bugs.launchpad.net/bugs/1950144 > > 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> > Link: https://lore.kernel.org/r/20211011053921.1198936-1-pasic@linux.ibm.com > Signed-off-by: Michael S. Tsirkin <mst@redhat.com> > (cherry picked from commit 2f9a174f918e29608564c7a4e8329893ab604fb4) > Signed-off-by: Frank Heimes <frank.heimes@canonical.com> > --- > drivers/virtio/virtio.c | 11 +++++++++++ > 1 file changed, 11 insertions(+) > Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com> Best regards, Krzysztof
diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c index 59a05f1b8105..91627e744326 100644 --- a/drivers/virtio/virtio.c +++ b/drivers/virtio/virtio.c @@ -225,6 +225,17 @@ 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