Re: [PATCH v1 1/7] virtio-gpu api: add blob userptr resource

["Huang, Honglei1" <Honglei1.Huang@amd.com>]

On 2025/3/18 9:37, Gurchetan Singh wrote:
> 
> 
> On Thu, Mar 6, 2025 at 2:52 AM Huang, Honglei1 <Honglei1.Huang@amd.com 
> <mailto:Honglei1.Huang@amd.com>> wrote:
> 
> 
>     On 2025/3/1 5:21, Demi Marie Obenour wrote:
>      > On 2/28/25 12:36 AM, Honglei Huang wrote:
>      >> From: Honglei Huang <Honglei1.Huang@amd.com
>     <mailto:Honglei1.Huang@amd.com>>
>      >>
>      >> Add a new resource for blob resource, called userptr, used for let
>      >> host access guest user space memory, to acquire buffer based userptr
>      >> feature in virtio GPU.
>      >>
>      >> - The capset VIRTIO_GPU_CAPSET_HSAKMT used for context init,
>      >> in this series patches only HSAKMT context can use the userptr
>      >> feature. HSAKMT is a GPU compute library in HSA stack, like
>      >> the role libdrm in mesa stack.
>      >
>      > Userptr should not be limited to HSMKMT contexts.  Userptr can
>      > accelerate shm buffers by avoiding a copy from guest to host, and
>      > it can be implemented using grant tables on Xen.
> 
>     Yes, I totally agree userptr can accelerate shm buffers, but I currently
>     don't know if there are any other projects working on similar features,
>     or if maintainers have any opinions or better ways to implement them, so
>     I temporarily limit this feature to HSAKMT context only.
> 
>     I am waiting for everyone's opinions, please provide your thoughts.
> 
> 
> I wonder if you can emulate userptr using udmabuf on the host-side?
> 
> Essentially for the guest, it'll be a malloc'ed memory, which means a 
> guest sg list.  We can convert the guest sg-list to udmabuf using well- 
> known mechanisms on the host side.  I hope amdkfd can operate on dma- 
> bufs too?
> 
> I do such a feature that would have a more generic utility outside of 
> HSAKMT contexts and not rely on Xen-specific grant tables  ... 
> checkout VIRTIO_GPU_BLOB_FLAG_CREATE_GUEST_HANDLE in crosvm for an example.

I think userptr can be emulated by udmabuf if the sg list is from guest 
user sapce and not moveable, amdkfd can operate dma-buf, and the userptr 
feature can be achieved by partial functions of dma-buf as far as I can see.

But the udmabuf need managed by UMD, this will result in some 
performance loss, and we are planning to implement virtio GPU userptr 
using hmm_rang_fault, udmabuf solution may be completely different from 
this one.

I saw the feature VIRTIO_GPU_BLOB_FLAG_CREATE_GUEST_HANDLE is used by 
the crosvm in beta branch, but I can not find the kernel virtio gpu 
part, how to access it? Maybe I can try or help.

> 
> 
> 
>     Regards,
>     Honglei
>