Re: [RFC PATCH 3/3] drm/virtio: implement blob userptr resource object

Huang, Honglei1
2024-12-27T02:24:49Z

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.

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