Unmapping a blob changes the memory map, which is protected with RCU.
RCU is designed to minimize the read-side overhead at the cost of
reclamation delay. While this design usually makes sense, it is
problematic when unmapping a blob because the operation blocks all
virtio-gpu commands and causes perceivable disruption.

Minimize such the disruption with force_rcu(), which minimizes the
reclamation delay at the cost of a read-side overhead.

Signed-off-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
---
 hw/display/virtio-gpu-virgl.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/hw/display/virtio-gpu-virgl.c b/hw/display/virtio-gpu-virgl.c
index 07f6355ad62e..71cde671c193 100644
--- a/hw/display/virtio-gpu-virgl.c
+++ b/hw/display/virtio-gpu-virgl.c
@@ -187,6 +187,7 @@ virtio_gpu_virgl_unmap_resource_blob(VirtIOGPU *g,
         memory_region_set_enabled(mr, false);
         memory_region_del_subregion(&b->hostmem, mr);
         object_unparent(OBJECT(mr));
+        force_rcu();
     }
 
     return 0;

-- 
2.51.0

On 10/29/25 09:12, Akihiko Odaki wrote:
> Unmapping a blob changes the memory map, which is protected with RCU.
> RCU is designed to minimize the read-side overhead at the cost of
> reclamation delay. While this design usually makes sense, it is
> problematic when unmapping a blob because the operation blocks all
> virtio-gpu commands and causes perceivable disruption.
> 
> Minimize such the disruption with force_rcu(), which minimizes the
> reclamation delay at the cost of a read-side overhead.
> 
> Signed-off-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
> ---
>  hw/display/virtio-gpu-virgl.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/hw/display/virtio-gpu-virgl.c b/hw/display/virtio-gpu-virgl.c
> index 07f6355ad62e..71cde671c193 100644
> --- a/hw/display/virtio-gpu-virgl.c
> +++ b/hw/display/virtio-gpu-virgl.c
> @@ -187,6 +187,7 @@ virtio_gpu_virgl_unmap_resource_blob(VirtIOGPU *g,
>          memory_region_set_enabled(mr, false);
>          memory_region_del_subregion(&b->hostmem, mr);
>          object_unparent(OBJECT(mr));
> +        force_rcu();
>      }
>  
>      return 0;
> 

Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>

-- 
Best regards,
Dmitry