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

["Huang, Honglei1" <Honglei1.Huang@amd.com>]

[-- Attachment #1: Type: text/plain, Size: 4221 bytes --]


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)", 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.
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

[-- Attachment #2: Type: text/html, Size: 6835 bytes --]