Re: [PATCH] virtio/s390: implement virtio-ccw revision 2 correctly

[Cornelia Huck <cohuck@redhat.com>]

On Fri, 12 Feb 2021 18:04:11 +0100
Cornelia Huck <cohuck@redhat.com> wrote:

> CCW_CMD_READ_STATUS was introduced with revision 2 of virtio-ccw,
> and drivers should only rely on it being implemented when they
> negotiated at least that revision with the device.
> 
> However, virtio_ccw_get_status() issued READ_STATUS for any
> device operating at least at revision 1. If the device accepts
> READ_STATUS regardless of the negotiated revision (which it is
> free to do),

So, looking at the standard again, the device is actually required to
reject the READ_STATUS if only rev 1 had been negotiated... regardless
of that, I don't think we should change QEMU's behaviour, as it would
affect existing guests (they would lose access to the status bits as
observed by the device, including DEVICE_NEEDS_RESET.)

> everything works as intended; a device rejecting the
> command should also be handled gracefully. For correctness, we
> should really limit the command to revision 2 or higher, though.
> 
> We also negotiated the revision to at most 1, as we never bumped
> the maximum revision; let's do that now.
> 
> Fixes: 7d3ce5ab9430 ("virtio/s390: support READ_STATUS command for virtio-ccw")
> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> ---
> 
> QEMU does not fence off READ_STATUS for revisions < 2, which is probably
> why we never noticed this. I'm not aware of other hypervisors that do
> fence it off, nor any that cannot deal properly with an unknown command.
> 
> Not sure whether this is stable worthy?

Maybe it is, given the MUST reject clause in the standard?

> 
> ---
>  drivers/s390/virtio/virtio_ccw.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/s390/virtio/virtio_ccw.c b/drivers/s390/virtio/virtio_ccw.c
> index 5730572b52cd..54e686dca6de 100644
> --- a/drivers/s390/virtio/virtio_ccw.c
> +++ b/drivers/s390/virtio/virtio_ccw.c
> @@ -117,7 +117,7 @@ struct virtio_rev_info {
>  };
>  
>  /* the highest virtio-ccw revision we support */
> -#define VIRTIO_CCW_REV_MAX 1
> +#define VIRTIO_CCW_REV_MAX 2
>  
>  struct virtio_ccw_vq_info {
>  	struct virtqueue *vq;
> @@ -952,7 +952,7 @@ static u8 virtio_ccw_get_status(struct virtio_device *vdev)
>  	u8 old_status = vcdev->dma_area->status;
>  	struct ccw1 *ccw;
>  
> -	if (vcdev->revision < 1)
> +	if (vcdev->revision < 2)
>  		return vcdev->dma_area->status;
>  
>  	ccw = ccw_device_dma_zalloc(vcdev->cdev, sizeof(*ccw));