Virtualization - Feb 2021 - [PATCH v2] virtio/s390: implement virtio-ccw revision 2 correctly

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 some
implementations like QEMU do, even though the spec currently does
not allow it), everything works as intended. While a device
rejecting the command should also be handled gracefully, we will
not be able to see any changes the device makes to the status,
such as setting NEEDS_RESET or setting the status to zero after
a completed reset.

We negotiated the revision to at most 1, as we never bumped the
maximum revision; let's do that now and properly send READ_STATUS
only if we are operating at least at revision 2.

Cc: stable at vger.kernel.org
Fixes: 7d3ce5ab9430 ("virtio/s390: support READ_STATUS command for
virtio-ccw")
Reviewed-by: Halil Pasic <pasic at linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck at redhat.com>
---

v1->v2:
  tweak patch description and cc:stable

---
 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));
-- 
2.26.2

I was thinking of queuing this, but maybe it is quicker to pick it into
the s390 tree directly and save us the extra pull request dance?
Especially as this is a stable-worthy bugfix.

On Tue, 16 Feb 2021 12:06:45 +0100
Cornelia Huck <cohuck at 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 some
> implementations like QEMU do, even though the spec currently does
> not allow it), everything works as intended. While a device
> rejecting the command should also be handled gracefully, we will
> not be able to see any changes the device makes to the status,
> such as setting NEEDS_RESET or setting the status to zero after
> a completed reset.
> 
> We negotiated the revision to at most 1, as we never bumped the
> maximum revision; let's do that now and properly send READ_STATUS
> only if we are operating at least at revision 2.
> 
> Cc: stable at vger.kernel.org
> Fixes: 7d3ce5ab9430 ("virtio/s390: support READ_STATUS command for
virtio-ccw")
> Reviewed-by: Halil Pasic <pasic at linux.ibm.com>
> Signed-off-by: Cornelia Huck <cohuck at redhat.com>
> ---
> 
> v1->v2:
>   tweak patch description and cc:stable
> 
> ---
>  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));