Re: [RFC PATCH 3/3] drm/virtio: drm_gem_plane_helper_prepare_fb for obj synchronization

["Kim, Dongwon" <dongwon.kim@intel.com>]

On 8/17/2023 7:33 PM, Dmitry Osipenko wrote:
> On 7/13/23 01:44, Dongwon Kim wrote:
>> This helper is needed for framebuffer synchronization. Old framebuffer data
>> is often displayed on the guest display without this helper.
>>
>> Cc: Gerd Hoffmann <kraxel@redhat.com>
>> Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
>> Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
>> ---
>>   drivers/gpu/drm/virtio/virtgpu_plane.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/virtio/virtgpu_plane.c b/drivers/gpu/drm/virtio/virtgpu_plane.c
>> index a063f06ab6c5..e197299489ce 100644
>> --- a/drivers/gpu/drm/virtio/virtgpu_plane.c
>> +++ b/drivers/gpu/drm/virtio/virtgpu_plane.c
>> @@ -26,6 +26,7 @@
>>   #include <drm/drm_atomic_helper.h>
>>   #include <drm/drm_damage_helper.h>
>>   #include <drm/drm_fourcc.h>
>> +#include <drm/drm_gem_atomic_helper.h>
>>   
>>   #include "virtgpu_drv.h"
>>   
>> @@ -271,6 +272,9 @@ static int virtio_gpu_plane_prepare_fb(struct drm_plane *plane,
>>   	vgfb = to_virtio_gpu_framebuffer(new_state->fb);
>>   	vgplane_st = to_virtio_gpu_plane_state(new_state);
>>   	bo = gem_to_virtio_gpu_obj(vgfb->base.obj[0]);
>> +
>> +	drm_gem_plane_helper_prepare_fb(plane, new_state);
> The implicit display BO sync should happen on a host side, unless you're
> rendering with Venus and then displaying with virgl. Doing it on guest
> side should be a major performance hit. Please provide a complete
> description of your setup: what VMM you use, config options, what tests
> you're running.

We use virtio-gpu as a kms device while using i915 as the render device in our setup.
And we use QEMU as VMM. Virtio-gpu driver flushes the scanout to QEMU as a blob resource
(reference to the buffer). QEMU then creates a dmabuf using udmabuf for the blob
then renders it as a texture using OGL. The test I ran is simple. Just starting terminal
app and typing things to see if there is any frame regression. I believe this helper is
required since the BO on the guest is basically dmabuf that is being shared between i915
and virtio-gpu driver. I didn't think about the performance impact. If the impact is
too much and that is not acceptable, is there any other suggestions or some tests I can try?

Thanks!