On Mon, Jan 20, 2025 at 05:54:10PM +0800, Huang, Honglei1 wrote:
On 2024/12/27 10:02, Huang, Honglei1 wrote:
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@xxxxxxx>
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.
I kinda ignored this one, because from my point of view it's all clear.
But I guess better to elaborate some more.
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)", thenkernel 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 notifierin virtio-GPU driver and free the
corresponding data in host
by some virtio CMDs as far as I can see.
Yes that's what I meant that you need real svm/hmm here. You cannot fake
the amdkfd/hsakmt model with pin_user_pages, it fundamentally falls apart.
One case is overcommit, where userspace malloc() a huge amount of virtual
address space, registers it with the gpu, but only uses fairly little of
it. If you pin that all, you run out of memory or at least thrash
performance.
For the hsa/kfd model, you must use hmm+mmu_notifier, or you're breaking
the uapi contract.
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.
I forgot the exact details, but when I looked amdkfd was taking too big
locks in the migration paths, which results in deadlocks. Those were
worked around by taking even bigger locks, the mmap_write_lock. But that's
a design issue with amdkfd, not something fundamental.
hmm only needs the mmu_notifier callbacks to work, meaning either context
preemption and restarting, or tlb invalidation and gpu page fault
handling. Those you can forward between guest and host with no issues, and
with hw support like pasid/ats in iommu this already happens.
Note that the drm_gpusvm library that's under discussion for xe had the
same issue in version 1 of relying on mmap_write_lock to paper over design
issues. But the recent versions should be fixed. Would be really good to
look at all that. And then probably do a full redo of the svm support for
amd in amdgpu.ko using all these new helper libraries, because my personal
take is that fixing amdkfd is probably more work than just writing new
clean code.
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
Hi Sima,
I modified the code, remove all the MMU nitifior and use pin_user_pages_fast
only. Under this implementation userptr fully
managed by UMD. We did a performance test, it decreased by 30% in
OpenCL stack in Geekbench6 benmark.
We use AMD V2000 for test:
use MMU notifior + pin_user_pages:
near 13000 score: https://browser.geekbench.com/v6/compute/3257793
use pin_user_pages only:
near 10000 socre: https://browser.geekbench.com/v6/compute/3496228
The code is under clean up, I will send out later.
pin_user_pages is fundamentally broken for the hsa memory model, no amount
of benchmarking different flavors of it against each another will change
that.
And I found a another thing, it seems like in intel i915 userptr
implementation, the pin_user_pages is also used in MMU notifior.
Code path is: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/drm/i915/gem/i915_gem_userptr.c?h=v6.13#:~:text=ret%20%3D%20pin_user_pages_fast(obj%2D%3Euserptr.ptr%20%2B%20pinned%20*%20PAGE_SIZE%2C
Yeah i915-gem isn't great code and shouldn't be used as example for
anything. This is Dave&me asked Xe developers to create a lot of the
infrastructure in drm libraries, to make sure we have a solid design here
and not what i915-gem did.
Also note that i915-gem userptr is for buffer based userptr, like for
VK_KHR_external_memory if I remember correctly. This is not the hsa memory
model at all.
Patch set: https://lore.kernel.org/all/159353552439.22701.14005121342739071590@xxxxxxxxxxxxxxxxxxxxxx/T/
https://patchwork.kernel.org/project/intel-gfx/patch/20210323155059.628690-17-maarten.lankhorst@xxxxxxxxxxxxxxx/#24064663
And I didn't find the hmm_range_fault code path, maybe I missed it?
i915-gem never supported the svm/hsa memory model in upstream, so yeah
it's not there. For drm/xe see the work from Matt Brost that's currently
under discussion on dri-devel for what it should all look like.
Cheers, Sima
Regards,
Honglei
