Using "-device virtio-gpu,blob=true" currently does not work on big
endian hosts (like s390x). The guest kernel prints an error message
like:

 [drm:virtio_gpu_dequeue_ctrl_func [virtio_gpu]] *ERROR* response 0x1200 (command 0x10c)

and the display stays black. When running QEMU with "-d guest_errors",
it shows an error message like this:

 virtio_gpu_create_mapping_iov: nr_entries is too big (83886080 > 16384)

which indicates that this value has not been properly byte-swapped.
And indeed, the virtio_gpu_create_blob_bswap() function (that should
swap the fields in the related structure) fails to swap some of the
entries. After correctly swapping all missing values here, too, the
virtio-gpu device is now also working with blob=true on s390x hosts.

Fixes: e0933d91b1 ("virtio-gpu: Add virtio_gpu_resource_create_blob")
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2230469
Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 include/hw/virtio/virtio-gpu-bswap.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/hw/virtio/virtio-gpu-bswap.h b/include/hw/virtio/virtio-gpu-bswap.h
index 9124108485..637a0585d0 100644
--- a/include/hw/virtio/virtio-gpu-bswap.h
+++ b/include/hw/virtio/virtio-gpu-bswap.h
@@ -63,7 +63,10 @@ virtio_gpu_create_blob_bswap(struct virtio_gpu_resource_create_blob *cblob)
 {
     virtio_gpu_ctrl_hdr_bswap(&cblob->hdr);
     le32_to_cpus(&cblob->resource_id);
+    le32_to_cpus(&cblob->blob_mem);
     le32_to_cpus(&cblob->blob_flags);
+    le32_to_cpus(&cblob->nr_entries);
+    le64_to_cpus(&cblob->blob_id);
     le64_to_cpus(&cblob->size);
 }
 
-- 
2.39.3

On Tue, Aug 15, 2023 at 4:20 PM Thomas Huth <thuth@redhat.com> wrote:
>
> Using "-device virtio-gpu,blob=true" currently does not work on big
> endian hosts (like s390x). The guest kernel prints an error message
> like:
>
>  [drm:virtio_gpu_dequeue_ctrl_func [virtio_gpu]] *ERROR* response 0x1200 (command 0x10c)
>
> and the display stays black. When running QEMU with "-d guest_errors",
> it shows an error message like this:
>
>  virtio_gpu_create_mapping_iov: nr_entries is too big (83886080 > 16384)
>
> which indicates that this value has not been properly byte-swapped.
> And indeed, the virtio_gpu_create_blob_bswap() function (that should
> swap the fields in the related structure) fails to swap some of the
> entries. After correctly swapping all missing values here, too, the
> virtio-gpu device is now also working with blob=true on s390x hosts.
>
> Fixes: e0933d91b1 ("virtio-gpu: Add virtio_gpu_resource_create_blob")
> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2230469
> Signed-off-by: Thomas Huth <thuth@redhat.com>

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>

> ---
>  include/hw/virtio/virtio-gpu-bswap.h | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/include/hw/virtio/virtio-gpu-bswap.h b/include/hw/virtio/virtio-gpu-bswap.h
> index 9124108485..637a0585d0 100644
> --- a/include/hw/virtio/virtio-gpu-bswap.h
> +++ b/include/hw/virtio/virtio-gpu-bswap.h
> @@ -63,7 +63,10 @@ virtio_gpu_create_blob_bswap(struct virtio_gpu_resource_create_blob *cblob)
>  {
>      virtio_gpu_ctrl_hdr_bswap(&cblob->hdr);
>      le32_to_cpus(&cblob->resource_id);
> +    le32_to_cpus(&cblob->blob_mem);
>      le32_to_cpus(&cblob->blob_flags);
> +    le32_to_cpus(&cblob->nr_entries);
> +    le64_to_cpus(&cblob->blob_id);
>      le64_to_cpus(&cblob->size);
>  }
>
> --
> 2.39.3
>