On 2024/12/22 9:59, Demi Marie Obenour wrote:
On 12/20/24 10:35 AM, Simona Vetter wrote:

On Fri, Dec 20, 2024 at 06:04:09PM +0800, Honglei Huang wrote:

From: Honglei Huang <Honglei1.Huang@amd.com>

A virtio-gpu userptr is based on HMM notifier.
Used for let host access guest userspace memory and
notice the change of userspace memory.
This series patches are in very beginning state,
User space are pinned currently to ensure the host
device memory operations are correct.
The free and unmap operations for userspace can be
handled by MMU notifier this is a simple and basice
SVM feature for this series patches.
The physical PFNS update operations is splited into
two OPs in here. The evicted memories won't be used
anymore but remap into host again to achieve same
effect with hmm_rang_fault.

So in my opinion there are two ways to implement userptr that make sense:

- pinned userptr with pin_user_pages(FOLL_LONGTERM). there is not mmu
  notifier

- unpinnned userptr where you entirely rely on userptr and do not hold any
  page references or page pins at all, for full SVM integration. This
  should use hmm_range_fault ideally, since that's the version that
  doesn't ever grab any page reference pins.

All the in-between variants are imo really bad hacks, whether they hold a
page reference or a temporary page pin (which seems to be what you're
doing here). In much older kernels there was some justification for them,
because strange stuff happened over fork(), but with FOLL_LONGTERM this is
now all sorted out. So there's really only fully pinned, or true svm left
as clean design choices imo.

With that background, why does pin_user_pages(FOLL_LONGTERM) not work for
you?

+1 on using FOLL_LONGTERM.  Fully dynamic memory management has a huge cost
in complexity that pinning everything avoids.  Furthermore, this avoids the
host having to take action in response to guest memory reclaim requests.
This avoids additional complexity (and thus attack surface) on the host side.
Furthermore, since this is for ROCm and not for graphics, I am less concerned
about supporting systems that require swappable GPU VRAM.
Hi Sima and Demi, I totally agree the flag FOLL_LONGTERM is needed, I will add it in next version. And for the first pin variants implementation, the MMU notifier is also needed I think.
Cause the userptr feature in UMD generally used like this: the registering of userptr
always is explicitly invoked by user code like "registerMemoryToGPU(userptrAddr, ...)",
but for the userptr release/free, there is no explicit API for it, at least in hsakmt/KFD stack.
User just need call system call "free(userptrAddr)", then kernel driver will release the userptr
by MMU notifier callback.Virtio-GPU has no other way to know if user has been free the userptr
except for MMU notifior.And in UMD theres is no way to get the free() operation is invoked by user.
the only way is use MMU notifier in virtio-GPU driver and free the corresponding data in host
by some virtio CMDs as far as I can see.
And for the second way that is use hmm_range_fault, there is a predictable issues as far as I can see, at least in hsakmt/KFD stack. That is the memory may migrate when GPU/device is working. In bare metal, when memory is migrating KFD driver will pause the compute work of the device in mmap_wirte_lock then use hmm_range_fault to remap the migrated/evicted memories to GPU then restore the compute work of device to ensure the correction of the data. But in virtio-GPU driver the migration happen in guest kernel, the evict mmu notifier callback happens in guest, a virtio CMD can be used for notify host but as lack of mmap_write_lock protection in host kernel, host will hold invalid data for a short period of time, this may lead to some issues. And it is hard to fix as far as I can see.
Finally I will extract some APIs into helper according to your request, and I will refactor the whole userptr
implementation, use some callbacks in page getting path, let the pin method and hmm_range_fault can be choiced
in this series patches.

Regards,
Honglei