On Fri, Jul 10, 2020 at 4:31 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> Page reporting features were never supported by legacy hypervisors.
> Supporting them poses a problem: should we use native endian-ness (like
> current code assumes)? Or little endian-ness like the virtio spec says?
> Rather than try to figure out, and since results of
> incorrect endian-ness are dire, let's just block this configuration.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
So I am not sure about the patch description. In the case of page
poison and free page reporting I don't think we are defining anything
that doesn't already have a definition of how to use in legacy.
Specifically the virtio_balloon_config is already defined as having
all fields as little endian in legacy mode, and there is a definition
for all of the fields in a virtqueue and how they behave in legacy
mode.
As far as I can see the only item that may be an issue is the command
ID being supplied via the virtqueue for free page hinting, which
appears to be in native endian-ness. Otherwise it would have fallen
into the same category since it is making use of virtio_balloon_config
and a virtqueue for supplying the page location and length.
> ---
> drivers/virtio/virtio_balloon.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 5d4b891bf84f..b9bc03345157 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -1107,6 +1107,15 @@ static int virtballoon_restore(struct virtio_device *vdev)
>
> static int virtballoon_validate(struct virtio_device *vdev)
> {
> + /*
> + * Legacy devices never specified how modern features should behave.
> + * E.g. which endian-ness to use? Better not to assume anything.
> + */
> + if (!virtio_has_feature(vdev, VIRTIO_F_VERSION_1)) {
> + __virtio_clear_bit(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT);
> + __virtio_clear_bit(vdev, VIRTIO_BALLOON_F_PAGE_POISON);
> + __virtio_clear_bit(vdev, VIRTIO_BALLOON_F_REPORTING);
> + }
> /*
> * Inform the hypervisor that our pages are poisoned or
> * initialized. If we cannot do that then we should disable
The patch content itself I am fine with since odds are nobody would
expect to use these features with a legacy device.
Acked-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
