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

[Huang, Honglei1]

On 2025/3/18 19:25, Huang, Honglei1 wrote:
> On 2025/3/18 9:37, Gurchetan Singh wrote:
> > On Thu, Mar 6, 2025 at 2:52 AM Huang, Honglei1 <Honglei1.Huang@xxxxxxx 
> > <mailto:Honglei1.Huang@xxxxxxx>> 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@xxxxxxx
> >     <mailto:Honglei1.Huang@xxxxxxx>>
> >      >>
> >      >> 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.

Hi Gurchetan,

	I found VIRTIO_GPU_BLOB_FLAG_CREATE_GUEST_HANDLE has been added into 
crosvm 4 years ago. But I can not find the corresponding kernel 
implementation, also can not find in chromeos kernel:
https://chromium.googlesource.com/chromiumos/third_party/kernel/.
	And it seems like the VIRTIO_GPU_BLOB_FLAG_CREATE_GUEST_HANDLE only 
support the system memory according to the commit message, but I can see 
the virtio has plan to support guest 
heap:https://github.com/google/crosvm/blob/dee7838993bae90cad49d5595b958cfe4c28b58d/devices/src/virtio/gpu/virtio_gpu.rs#L1056. 

	Can I create new virtio gpu patches for adding heap supported virtio 
gpu udmabuf?

Regards,
Honglei

> >     Regards,
> >     Honglei