Re: [Qemu-devel] [PATCH 3/4] virtio-ccw: feature bits > 31 handling

[David Hildenbrand <dahi@linux.vnet.ibm.com>]

> We currently switch off the VERSION_1 feature bit if the guest has
> not negotiated at least revision 1. As no feature bits beyond 31 are
> valid however unless VERSION_1 has been negotiated, make sure that
> legacy guests never see a feature bit beyond 31.
> 
> Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
> ---
>  hw/s390x/virtio-ccw.c | 21 ++++++++-------------
>  1 file changed, 8 insertions(+), 13 deletions(-)
> 
> diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
> index 85e2a5d..eed7b3e 100644
> --- a/hw/s390x/virtio-ccw.c
> +++ b/hw/s390x/virtio-ccw.c
> @@ -468,15 +468,12 @@ static int virtio_ccw_cb(SubchDev *sch, CCW1 ccw)
>                                                  NULL);
>              if (features.index == 0) {
>                  features.features = (uint32_t)vdev->host_features;
> -            } else if (features.index == 1) {
> -                features.features = (uint32_t)(vdev->host_features >> 32);
> +            } else if ((features.index == 1) && (dev->revision >= 1)) {
>                  /*
> -                 * Don't offer version 1 to the guest if it did not
> -                 * negotiate at least revision 1.
> +                 * Only offer feature bits beyond 31 if the guest has
> +                 * negotiated at least revision 1.
>                   */
> -                if (dev->revision <= 0) {
> -                    features.features &= ~(1 << (VIRTIO_F_VERSION_1 - 32));
> -                }
> +                features.features = (uint32_t)(vdev->host_features >> 32);
>              } else {
>                  /* Return zeroes if the guest supports more feature bits. */
>                  features.features = 0;
> @@ -515,14 +512,12 @@ static int virtio_ccw_cb(SubchDev *sch, CCW1 ccw)
>                  virtio_set_features(vdev,
>                                      (vdev->guest_features & 0xffffffff00000000ULL) |
>                                      features.features);
> -            } else if (features.index == 1) {
> +            } else if ((features.index == 1) && (dev->revision >= 1)) {
>                  /*
> -                 * The guest should not set version 1 if it didn't
> -                 * negotiate a revision >= 1.
> +                 * If the guest did not negotiate at least revision 1,
> +                 * we did not offer it any feature bits beyond 31. Such a
> +                 * guest passing us any bit here is therefore buggy.
>                   */
> -                if (dev->revision <= 0) {
> -                    features.features &= ~(1 << (VIRTIO_F_VERSION_1 - 32));
> -                }
>                  virtio_set_features(vdev,
>                                      (vdev->guest_features & 0x00000000ffffffffULL) |
>                                      ((uint64_t)features.features << 32));

Looks sane to me!

Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>

David