Re: [PATCH 2/2] pc-bios/s390-ccw/virtio: Read device config after feature negotiation

[Cornelia Huck <cohuck@redhat.com>]

On Thu, Jun 23 2022, Thomas Huth <thuth@redhat.com> wrote:

> Feature negotiation should be done first, since some fields in the
> config area can depend on the negotiated features and thus should
> rather be read afterwards.

I suppose we don't negotiate any features that might affect the size of
the config space? Anyway, restricting ourselves to the minimum length
should be fine.

>
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  pc-bios/s390-ccw/virtio.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/pc-bios/s390-ccw/virtio.c b/pc-bios/s390-ccw/virtio.c
> index 4e85a2eb82..0e92e994df 100644
> --- a/pc-bios/s390-ccw/virtio.c
> +++ b/pc-bios/s390-ccw/virtio.c
> @@ -262,10 +262,6 @@ void virtio_setup_ccw(VDev *vdev)
>      rc = run_ccw(vdev, CCW_CMD_WRITE_STATUS, &status, sizeof(status), false);
>      IPL_assert(rc == 0, "Could not write DRIVER status to host");
>  
> -    IPL_assert(
> -        run_ccw(vdev, CCW_CMD_READ_CONF, &vdev->config, cfg_size, false) == 0,
> -       "Could not get block device configuration");
> -
>      /* Feature negotiation */
>      for (i = 0; i < ARRAY_SIZE(vdev->guest_features); i++) {
>          feats.features = 0;
> @@ -278,6 +274,9 @@ void virtio_setup_ccw(VDev *vdev)
>          IPL_assert(rc == 0, "Could not set features bits");
>      }
>  
> +    rc = run_ccw(vdev, CCW_CMD_READ_CONF, &vdev->config, cfg_size, false);
> +    IPL_assert(rc == 0, "Could not get block device configuration");

Since you move this anyway: s/block device/boot device/ ?

> +
>      for (i = 0; i < vdev->nr_vqs; i++) {
>          VqInfo info = {
>              .queue = (unsigned long long) ring_area + (i * VIRTIO_RING_SIZE),

Related to feature negotiation: It seems that the bios currently
supports legacy virtio only, doesn't it? It's probably fine for now, but
there might be a way in the future to disable legacy virtio for all
devices in QEMU, so we'll probably want to add virtio-1 support some
day.