On Fri, 14 Jan 2022 15:09:14 -0500
"Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
> 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@xxxxxxxxxxxxxxx
> Fixes: 404123c2db79 ("virtio: allow drivers to validate features")
> Fixes: 2f9a174f918e ("virtio: write back F_VERSION_1 before validate")
> Cc: "Halil Pasic" <pasic@xxxxxxxxxxxxx>
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
>
> 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!
Hi Michael!
I was just about to have a look into this. But it does not apply
cleanly to Linus master (fetched a couple of minutes ago). I also tride
with d9679d0013a66849~1 but no luck. What is a suitable base for this
patch?
Regards,
Halil
> 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;
