On Wed, Oct 06 2021, Halil Pasic <pasic@xxxxxxxxxxxxx> 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: driver MAY read (but MUST NOT
> write) the device-specific configuration fields to check that it can
> support the device before setting FEATURES_OK.
Suggest to put the citations from the spec into quotes, so that they are
distinguishable from the rest of the text.
>
> 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 regression so let's try to address
s/regression/a regression/
> it.
Maybe mention what the regression is?
Also mention that we use this workaround for modern on BE only?
>
> Signed-off-by: Halil Pasic <pasic@xxxxxxxxxxxxx>
> 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@xxxxxxxxxx
> ---
> drivers/virtio/virtio.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> index 0a5b54034d4b..494cfecd3376 100644
> --- a/drivers/virtio/virtio.c
> +++ b/drivers/virtio/virtio.c
> @@ -239,6 +239,16 @@ 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 for these when needed.
"...to force LE config space accesses before FEATURES_OK for these when
needed (BE)."
?
> + */
> + if (drv->validate && !virtio_legacy_is_little_endian()
> + && BIT_ULL(VIRTIO_F_VERSION_1) & device_features) {
Nit: putting device_features first would read more naturally to me.
> + 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
Patch LGTM.
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