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 it.
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.
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 --- 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