Re: [v2 1/3] virtio-gpu: Add support for VIRTIO_GPU_BLOB_FLAG_USE_USERPTR flag

[Honglei Huang <honghuan@amd.com>]

On 2025/11/21 14:12, Akihiko Odaki wrote:
> 
> 
> On 2025/11/21 14:43, Honglei Huang wrote:
>>
>>
>> On 2025/11/21 13:28, Akihiko Odaki wrote:
>>> On 2025/11/21 14:21, Honglei Huang wrote:
>>>>
>>>>
>>>> On 2025/11/21 11:39, Akihiko Odaki wrote:
>>>>> On 2025/11/21 12:14, Honglei Huang wrote:
>>>>>>
>>>>>>
>>>>>> On 2025/11/21 10:56, Akihiko Odaki wrote:
>>>>>>> On 2025/11/21 11:47, Honglei Huang wrote:
>>>>>>>> Add support for the USE_USERPTR blob flag in virtio-gpu to enable
>>>>>>>> user pointer mapping for blob resources. This allows guest 
>>>>>>>> applications
>>>>>>>> to use user-allocated memory for GPU resources more efficiently.
>>>>>>>>
>>>>>>>> Changes include:
>>>>>>>> - Add VIRTIO_GPU_BLOB_FLAG_USE_USERPTR flag definition
>>>>>>>> - Enhance blob resource creation to handle userptr flag properly
>>>>>>>> - Remove arbitrary nr_entries limit (16384) in mapping creation
>>>>>>>> - Add conditional handling for userptr vs regular blob mapping
>>>>>>>
>>>>>>> I don't see the added conditional handling.
>>>>>>
>>>>>> Sorry, the additional handing is replaced by the fixing of value 
>>>>>> check.
>>>>>> I will correct this commit message in the next version.
>>>>>
>>>>> Not just the commit message, but it also questions the utility of 
>>>>> VIRTIO_GPU_BLOB_FLAG_USE_USERPTR and VIRTIO_GPU_F_RESOURCE_USERPTR. 
>>>>> Neither of them adds a new functionality. They should be dropped if 
>>>>> they are also replaced with the fix.
>>>>>
>>>>
>>>> Yes totally agreed, it is my mistaken, I shouldn't mix the code for 
>>>> fixing and the code for adding new features in one submission.
>>>>
>>>> Actually this patch set are for another components upstream test, 
>>>> for the sake of convenience, I have added both the fix and feature 
>>>> here,
>>>> that is a bad idea.
>>>>
>>>> Will split the fix part into previous thread.
>>>>
>>>> And for the check value fix thread, will send v4 as the final version.
>>>
>>> Splitting fixes and features is a good idea, but that's not what I 
>>> meant.
>>>
>>> What I pointed out is that, it seems that one of the "features" you 
>>> are adding, namely VIRTIO_GPU_F_RESOURCE_USERPTR does nothing at at 
>>> all. So I'm wondering if you forgot to add a real implementation or 
>>> the feature is just no longer necessary.
>>
>> Understood, actually the resource of flag 
>> VIRTIO_GPU_F_RESOURCE_USERPTR just reuses the feature of 
>> VIRTIO_GPU_BLOB_MEM_GUEST: using the virtio_gpu_create_mapping_iov 
>> function to map the iov from guest.
>>
>> In qemu, the handing of VIRTIO_GPU_F_RESOURCE_USERPTR and 
>> VIRTIO_GPU_BLOB_MEM_GUEST basically same.
>>
>> The VIRTIO_GPU_F_RESOURCE_USERPTR is from guest userspace, but the
>> VIRTIO_GPU_BLOB_MEM_GUEST comes from guest kernel.
>> So in VIRTIO kernel and virglrenderer they are different, see VIRTIO 
>> kerenl: [1], and virglrenderer: [2].
>>
>> May I need to change the organizational form of this patch set?
>>
>> [1]: https://lore.kernel.org/lkml/20251112074548.3718563-1- 
>> honglei1.huang@amd.com/
>> [2]: https://gitlab.freedesktop.org/virgl/virglrenderer/-/ 
>> merge_requests/1568/ 
>> diffs#14086999aaf57fc68a3d7d639ab280c3a2672430:~:text=if%20(args%2D%3Eblob_flags%20%26%20VIRGL_RENDERER_BLOB_FLAG_USE_USERPTR)%20%7B
> Why does virglrenderer need to distinguish userspace and kernel?

It seems like the blob resource in virglrenderer doesn't support the 
guest resource/iov resource, virglrenderer doesn't let guest kernel 
resource go into get_blob callback.

But some feature I am working on really want to let guest userptr 
resource get in get_blob callback.

So I use this flag to extend the feature of virglrenderer blob callback.

The different is:
guest kernel resource -> won't go to blob callback.
guest userptr resource -> go to blob callback.