linux-kernel - Re: [PATCH] drm/virtio: Allow importing prime buffers when 3D is enabled

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Message-ID: <1005ebfc-6f0e-477f-afae-2d42b44a5fa3@invisiblethingslab.com>
Date: Wed, 10 Dec 2025 19:11:01 -0300
From: Val Packett <val@...isiblethingslab.com>
To: Dmitry Osipenko <dmitry.osipenko@...labora.com>,
 David Airlie <airlied@...hat.com>, Gerd Hoffmann <kraxel@...hat.com>,
 Gurchetan Singh <gurchetansingh@...omium.org>, Chia-I Wu
 <olvaffe@...il.com>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 Simona Vetter <simona@...ll.ch>, Vivek Kasireddy <vivek.kasireddy@...el.com>
Cc: Dongwon Kim <dongwon.kim@...el.com>,
 Christian Koenig <christian.koenig@....com>,
 dri-devel@...ts.freedesktop.org, virtualization@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/virtio: Allow importing prime buffers when 3D is
 enabled


On 12/10/25 1:51 PM, Dmitry Osipenko wrote:
> Hi,
>
> On 12/10/25 18:39, Val Packett wrote:
>> This functionality was added for using a KMS-only virtgpu with a physical
>> (or SR-IOV) headless GPU in passthrough, but it should not be restricted
>> to KMS-only mode. It can be used with cross-domain to pass guest memfds
>> to the host compositor with zero copies (using udmabuf on both sides).
>>
>> Drop the check for the absence of virgl_3d to allow for more use cases.
>>
>> Fixes: ca77f27a2665 ("drm/virtio: Import prime buffers from other devices as guest blobs")
>> Signed-off-by: Val Packett <val@...isiblethingslab.com>
>> ---
>>
>> Hi. I couldn't find any comments on that line in the reviews (on patchwork), so I assume
>> there was never a specific technical reason for that check, just an abundance of caution?
>>
>> BTW, while here.. The drm_gem_prime_import "fallback" seems pretty much equivalent to
>> `return (-ENODEV)`, as drm_gem_prime_import(_dev) just translates the call to
>> gem_prime_import_sg_table which we don't use. Should it be replaced with `return (-ENODEV)`?
> Returning -ENODEV should break dmabuf self-importing where virtio-gpu
> driver export dmabuf and then imports to itself.

Hm, I don't think so because the self-import case (for when `buf->ops == 
&virtgpu_dmabuf_ops`) is handled right here, right above this check.

drm_gem_prime_import would handle the self-import for `buf->ops == 
&drm_gem_prime_dmabuf_ops` which shouldn't be the case since we have 
`virtgpu_dmabuf_ops`..

>> ---
>>   drivers/gpu/drm/virtio/virtgpu_prime.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/virtio/virtgpu_prime.c b/drivers/gpu/drm/virtio/virtgpu_prime.c
>> index ce49282198cb..2fedd5d3bd62 100644
>> --- a/drivers/gpu/drm/virtio/virtgpu_prime.c
>> +++ b/drivers/gpu/drm/virtio/virtgpu_prime.c
>> @@ -312,7 +312,7 @@ struct drm_gem_object *virtgpu_gem_prime_import(struct drm_device *dev,
>>   		}
>>   	}
>>   
>> -	if (!vgdev->has_resource_blob || vgdev->has_virgl_3d)
>> +	if (!vgdev->has_resource_blob)
>>   		return drm_gem_prime_import(dev, buf);
>>   
>>   	bo = kzalloc(sizeof(*bo), GFP_KERNEL);
> At a quick glance the change looks fine. Will take another stab after
> Holidays and merge if no problems will be spotted. Thanks.

Thanks!
~val