On 2026/01/16 19:32, Honglei Huang wrote:
On 2026/1/16 18:01, Akihiko Odaki wrote:
On 2026/01/16 18:39, Honglei Huang wrote:
On 2026/1/16 16:54, Akihiko Odaki wrote:
On 2026/01/16 16:20, Honglei Huang wrote:
On 2026/1/15 17:20, Akihiko Odaki wrote:
On 2026/01/15 16:58, Honglei Huang wrote:
From: Honglei Huang <[email protected]>
Hello,
This series adds virtio-gpu userptr support to enable ROCm native
context for compute workloads. The userptr feature allows the
host to
directly access guest userspace memory without memcpy overhead,
which is
essential for GPU compute performance.
The userptr implementation provides buffer-based zero-copy memory
access.
This approach pins guest userspace pages and exposes them to the
host
via scatter-gather tables, enabling efficient compute operations.
This description looks identical with what
VIRTIO_GPU_BLOB_MEM_HOST3D_GUEST does so there should be some
explanation how it makes difference.
I have already pointed out this when reviewing the QEMU
patches[1], but I note that here too, since QEMU is just a
middleman and this matter is better discussed by Linux and
virglrenderer developers.
[1] https://lore.kernel.org/qemu-devel/35a8add7-da49-4833-9e69-
[email protected]/
Thanks for raising this important point about the distinction between
VIRTGPU_BLOB_FLAG_USE_USERPTR and VIRTIO_GPU_BLOB_MEM_HOST3D_GUEST.
I might not have explained it clearly previously.
The key difference is memory ownership and lifecycle:
BLOB_MEM_HOST3D_GUEST:
- Kernel allocates memory (drm_gem_shmem_create)
- Userspace accesses via mmap(GEM_BO)
- Use case: Graphics resources (Vulkan/OpenGL)
BLOB_FLAG_USE_USERPTR:
- Userspace pre-allocates memory (malloc/mmap)
"Kernel allocates memory" and "userspace pre-allocates memory" is a
bit ambiguous phrasing. Either way, the userspace requests the
kernel to map memory with a system call, brk() or mmap().
They are different:
BLOB_MEM_HOST3D_GUEST (kernel-managed pages):
- Allocated via drm_gem_shmem_create() as GFP_KERNEL pages
- Kernel guarantees pages won't swap or migrate while GEM object
exists
- Physical addresses remain stable → safe for DMA
BLOB_FLAG_USE_USERPTR (userspace pages):
- From regular malloc/mmap - subject to MM policies
- Can be swapped, migrated, or compacted by kernel
- Requires FOLL_LONGTERM pinning to make DMA-safe
The device must treat them differently. Kernel-managed pages have
stable physical
addresses. Userspace pages need explicit pinning and the device must
be prepared
for potential invalidation.
This is why all compute drivers (amdgpu, i915, nouveau) implement
userptr - to
make arbitrary userspace allocations DMA-accessible while respecting
their different
page mobility characteristics.
And the drm already has a better frame work for it: SVM, and this
verions is a super simplified verion.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/
tree/ drivers/gpu/drm/
drm_gpusvm.c#:~:text=*%20GPU%20Shared%20Virtual%20Memory%20(GPU%20SVM)%20layer%20for%20the%20Direct%20Rendering%20Manager%20(DRM)
I referred to phrasing "kernel allocates" vs "userspace allocates".
Using GFP_KERNEL, swapping, migrating, or pinning is all what the
kernel does.
I am talking about the virtio gpu driver side, the virtio gpu driver
need handle those two type memory differently.
- Kernel only get existing pages
- Use case: Compute workloads (ROCm/CUDA) with large datasets, like
GPU needs load a big model file 10G+, UMD mmap the fd file, then
give the mmap ptr into userspace then driver do not need a another
copy.
But if the shmem is used, the userspace needs copy the file data
into a shmem mmap ptr there is a copy overhead.
Userptr:
file -> open/mmap -> userspace ptr -> driver
shmem:
user alloc shmem ──→ mmap shmem ──→ shmem userspace ptr -> driver
↑
│ copy
│
file ──→ open/mmap ──→ file userptr ──────────┘
For compute workloads, this matters significantly:
Without userptr: malloc(8GB) → alloc GEM BO → memcpy 8GB →
compute → memcpy 8GB back
With userptr: malloc(8GB) → create userptr BO → compute
(zero- copy)
Why don't you alloc GEM BO first and read the file into there?
Because that defeats the purpose of zero-copy.
With GEM-BO-first (what you suggest):
void *gembo = virtgpu_gem_create(10GB); // Allocate GEM buffer
void *model = mmap(..., model_file_fd, 0); // Map model file
memcpy(gembo, model, 10GB); // Copy 10GB - NOT zero-copy
munmap(model, 10GB);
gpu_compute(gembo);
Result: 10GB copy overhead + double memory usage during copy.
How about:
void *gembo = virtgpu_gem_create(10GB);
read(model_file_fd, gembo, 10GB);
I believe there is still memory copy in read operation
model_file_fd -> gembo, they have different physical pages,
but the userptr/SVM feature will access the model_file_fd physical pages
directly.
You can use O_DIRECT if you want.
Result: zero-copy + simpler code.
With userptr (zero-copy):
void *model = mmap(..., model_file_fd, 0); // Map model file
hsa_memory_register(model, 10GB); // Pin pages, create
userptr BO
gpu_compute(model); // GPU reads directly
from file pages
The explicit flag serves three purposes:
1. Although both send scatter-gather entries to host. The flag
makes the intent unambiguous.
Why will the host care?
The flag tells host this is a userptr, host side need handle it
specially.
Please provide the concrete requirement. What is the special handling
the host side needs to perform?
Every hardware has it own special API to handle userptr, for amdgpu ROCm
it is hsaKmtRegisterMemoryWithFlags.
On the host side, BLOB_MEM_HOST3D_GUEST will always result in a
userspace pointer. Below is how the address is translated:
1) (with the ioctl you are adding)
Guest kernel translates guest userspace pointer to guest PA.
2) (with IOMMU)
Guest kernel translates guest PA to device VA
3) The host VMM translates device VA to host userspace pointer
4) virglrenderer passes userspace pointer to the GPU API (ROCm)
BLOB_FLAG_USE_USERPTR tells 1) happened. But the succeeding process is
not affected by that.
2. Ensures consistency between flag and userptr address field.
Addresses are represented with the nr_entries and following struct
virtio_gpu_mem_entry entries, whenever
VIRTIO_GPU_CMD_RESOURCE_CREATE_BLOB or
VIRTIO_GPU_CMD_RESOURCE_ATTACH_BACKING is used. Having a special
flag introduces inconsistency.
For this part I am talking about the virito gpu guest UMD side, in
blob create io ctrl we need this flag to
check the userptr address and is it a read-only attribute:
if (rc_blob->blob_flags & VIRTGPU_BLOB_FLAG_USE_USERPTR) {
if (!rc_blob->userptr)
return -EINVAL;
} else {
if (rc_blob->userptr)
return -EINVAL;
if (rc_blob->blob_flags & VIRTGPU_BLOB_FLAG_USERPTR_RDONLY)
return -EINVAL;
}
I see. That shows VIRTGPU_BLOB_FLAG_USE_USERPTR is necessary for the
ioctl.
3. Future HMM support: There is a plan to upgrade userptr
implementation to use Heterogeneous Memory Management for better
GPU coherency and dynamic page migration. The flag provides a clean
path to future upgrade.
How will the upgrade path with the flag and the one without the flag
look like, and in what aspect the upgrade path with the flag is
"cleaner"?
As I mentioned above the userptr handling is different with shmem/GEM
BO.
All the above describes the guest-internal behavior. What about the
interaction between the guest and host? How will virtio as a guest-
host interface having VIRTIO_GPU_BLOB_FLAG_USE_USERPTR ease future
upgrade?
It depends on how we implement it, the current version is the simplest
implementation, similar to the implementation in Intel's i915.
If virtio side needs HMM to implement a SVM type userptr feature
I think VIRTIO_GPU_BLOB_FLAG_USE_USERPTR is must needed, stack needs to
know if it is a userptr resource, and to perform advanced operations
such as updating page tables, splitting BOs, etc.
Why do the device need to know if it is a userptr resource to perform
operations when the device always get device VAs?
I understand the concern about API complexity. I'll defer to the
virtio- gpu maintainers for the final decision on whether this
design is acceptable or if they prefer an alternative approach.
It is fine to have API complexity. The problem here is the lack of
clear motivation and documentation.
Another way to put this is: how will you explain the flag in the
virtio specification? It should say "the driver MAY/SHOULD/MUST do
something" and/or "the device MAY/SHOULD/MUST do something", and
then Linux and virglrenderer can implement the flag accordingly.
you're absolutely right that the specification should
be written in proper virtio spec language. The draft should be:
VIRTIO_GPU_BLOB_FLAG_USE_USERPTR:
Linux virtio driver requirements:
- MUST set userptr to valid guest userspace VA in
drm_virtgpu_resource_create_blob
- SHOULD keep VA mapping valid until resource destruction
- MUST pin pages or use HMM at blob creation time
These descriptions are not for the virtio specification. The virtio
specification describes the interaction between the driver and device.
These statements describe the interaction between the guest userspace
and the guest kernel.
Virglrenderer requirements:
- must use correspoonding API for userptr resource
What is the "corresponding API"?
It may can be:
**VIRTIO_GPU_BLOB_FLAG_USE_USERPTR specification:**
Driver requirements:
- MUST populate mem_entry[] with valid guest physical addresses of
pinned userspace pages
"Userspace" is a the guest-internal concepts and irrelevant with the
interaction between the driver and device.
- MUST set blob_mem to VIRTIO_GPU_BLOB_FLAG_USE_USERPTR when using this
flag
When should the driver use the flag?
- SHOULD keep pages pinned until VIRTIO_GPU_CMD_RESOURCE_UNREF
It is not a new requirement. The page must stay at the same position
whether VIRTIO_GPU_BLOB_FLAG_USE_USERPTR is used or not.
Device requirements:
- MUST establish IOMMU mappings using the provided iovec array with
specific API.(hsaKmtRegisterMemoryWithFlags for ROCm)
This should be also true even when VIRTIO_GPU_BLOB_FLAG_USE_USERPTR is
not set.
Really thanks for your comments, and I believe we need some input of
virito gpu maintainers.
VIRTIO_GPU_BLOB_FLAG_USE_USERPTR flag is a flag for how to use, and it
doen't conflict with VIRTGPU_BLOB_MEM_HOST3D_GUEST. Just like a resource
is used for VIRTGPU_BLOB_FLAG_USE_SHAREABLE but it can be a guest
resource or a host resource.
If we don't have VIRTIO_GPU_BLOB_FLAG_USE_USERPTR flag, we may have some
resource conflict in host side, guest kernel can use 'userptr' param to
identify. But in host side the 'userptr' param is lost, we only know it
is just a guest flag resource.
I still don't see why knowing it is a guest resource is insufficient for
the host.
Regards,
AKihiko Odaki
