The feature negotiation was designed in a way that makes it possible for devices to know which config fields will be accessed by drivers.
This is broken since commit 404123c2db79 ("virtio: allow drivers to validate features") with fallout in at least block and net. We have a partial work-around in commit 2f9a174f918e ("virtio: write back F_VERSION_1 before validate") which at least lets devices find out which format should config space have, but this is a partial fix: guests should not access config space without acknowledging features since otherwise we'll never be able to change the config space format.
As a side effect, this also reduces the amount of hypervisor accesses - we now only acknowledge features once unless we are clearing any features when validating.
Cc: stable@vger.kernel.org Fixes: 404123c2db79 ("virtio: allow drivers to validate features") Fixes: 2f9a174f918e ("virtio: write back F_VERSION_1 before validate") Cc: "Halil Pasic" pasic@linux.ibm.com Signed-off-by: Michael S. Tsirkin mst@redhat.com ---
Halil, I thought hard about our situation with transitional and today I finally thought of something I am happy with. Pls let me know what you think. Testing on big endian would also be much appreciated!
drivers/virtio/virtio.c | 31 +++++++++++++++++-------------- 1 file changed, 17 insertions(+), 14 deletions(-)
diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c index d891b0a354b0..2ed6e2451fd8 100644 --- a/drivers/virtio/virtio.c +++ b/drivers/virtio/virtio.c @@ -168,12 +168,10 @@ EXPORT_SYMBOL_GPL(virtio_add_status);
static int virtio_finalize_features(struct virtio_device *dev) { - int ret = dev->config->finalize_features(dev); unsigned status; + int ret;
might_sleep(); - if (ret) - return ret;
ret = arch_has_restricted_virtio_memory_access(); if (ret) { @@ -244,17 +242,6 @@ 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 @@ -265,10 +252,22 @@ static int virtio_dev_probe(struct device *_d) if (device_features & (1ULL << i)) __virtio_set_bit(dev, i);
+ err = dev->config->finalize_features(dev); + if (err) + goto err; + if (drv->validate) { + u64 features = dev->features; + err = drv->validate(dev); if (err) goto err; + + if (features != dev->features) { + err = dev->config->finalize_features(dev); + if (err) + goto err; + } }
err = virtio_finalize_features(dev); @@ -495,6 +494,10 @@ int virtio_device_restore(struct virtio_device *dev) /* We have a driver! */ virtio_add_status(dev, VIRTIO_CONFIG_S_DRIVER);
+ ret = dev->config->finalize_features(dev); + if (ret) + goto err; + ret = virtio_finalize_features(dev); if (ret) goto err;