Making drm_gpuvm work across gpu devices

Thu Feb 29 09:41:24 UTC 2024

Am 28.02.24 um 20:51 schrieb Zeng, Oak:
>
> The mail wasn’t indent/preface correctly. Manually format it.
>
> *From:*Christian König <christian.koenig at amd.com>
> *Sent:* Tuesday, February 27, 2024 1:54 AM
> *To:* Zeng, Oak <oak.zeng at intel.com>; Danilo Krummrich 
> <dakr at redhat.com>; Dave Airlie <airlied at redhat.com>; Daniel Vetter 
> <daniel at ffwll.ch>; Felix Kuehling <felix.kuehling at amd.com>; 
> jglisse at redhat.com
> *Cc:* Welty, Brian <brian.welty at intel.com>; 
> dri-devel at lists.freedesktop.org; intel-xe at lists.freedesktop.org; 
> Bommu, Krishnaiah <krishnaiah.bommu at intel.com>; Ghimiray, Himal Prasad 
> <himal.prasad.ghimiray at intel.com>; Thomas.Hellstrom at linux.intel.com; 
> Vishwanathapura, Niranjana <niranjana.vishwanathapura at intel.com>; 
> Brost, Matthew <matthew.brost at intel.com>; Gupta, saurabhg 
> <saurabhg.gupta at intel.com>
> *Subject:* Re: Making drm_gpuvm work across gpu devices
>
> Hi Oak,
>
> Am 23.02.24 um 21:12 schrieb Zeng, Oak:
>
>     Hi Christian,
>
>     I go back this old email to ask a question.
>
>
> sorry totally missed that one.
>
>     Quote from your email:
>
>     “Those ranges can then be used to implement the SVM feature
>     required for higher level APIs and not something you need at the
>     UAPI or even inside the low level kernel memory management.”
>
>     “SVM is a high level concept of OpenCL, Cuda, ROCm etc.. This
>     should not have any influence on the design of the kernel UAPI.”
>
>     There are two category of SVM:
>
>     1.driver svm allocator: this is implemented in user space,  i.g.,
>     cudaMallocManaged (cuda) or zeMemAllocShared (L0) or
>     clSVMAlloc(openCL). Intel already have gem_create/vm_bind in xekmd
>     and our umd implemented clSVMAlloc and zeMemAllocShared on top of
>     gem_create/vm_bind. Range A..B of the process address space is
>     mapped into a range C..D of the GPU address space, exactly as you
>     said.
>
>     2.system svm allocator:  This doesn’t introduce extra driver API
>     for memory allocation. Any valid CPU virtual address can be used
>     directly transparently in a GPU program without any extra driver
>     API call. Quote from kernel Documentation/vm/hmm.hst: “Any
>     application memory region (private anonymous, shared memory, or
>     regular file backed memory) can be used by a device transparently”
>     and “to share the address space by duplicating the CPU page table
>     in the device page table so the same address points to the same
>     physical memory for any valid main memory address in the process
>     address space”. In system svm allocator, we don’t need that A..B
>     C..D mapping.
>
>     It looks like you were talking of 1). Were you?
>
>
> No, even when you fully mirror the whole address space from a process 
> into the GPU you still need to enable this somehow with an IOCTL.
>
> And while enabling this you absolutely should specify to which part of 
> the address space this mirroring applies and where it maps to.
>
> */[Zeng, Oak] /*
>
> Lets say we have a hardware platform where both CPU and GPU support 
> 57bit(use it for example. The statement apply to any address range) 
> virtual address range, how do you decide “which part of the address 
> space this mirroring applies”? You have to mirror the whole address 
> space [0~2^57-1], do you? As you designed it, the gigantic 
> vm_bind/mirroring happens at the process initialization time, and at 
> that time, you don’t know which part of the address space will be used 
> for gpu program. Remember for system allocator, *any* valid CPU 
> address can be used for GPU program.  If you add an offset to 
> [0~2^57-1], you get an address out of 57bit address range. Is this a 
> valid concern?
>

Well you can perfectly mirror on demand. You just need something similar 
to userfaultfd() for the GPU. This way you don't need to mirror the full 
address space, but can rather work with large chunks created on demand, 
let's say 1GiB or something like that.

The virtual address space is basically just a hardware functionality to 
route memory accesses. While the mirroring approach is a very common use 
case for data-centers and high performance computing there are quite a 
number of different use cases which makes use of virtual address space 
in a non "standard" fashion. The native context approach for VMs is just 
one example, databases and emulators are another one.

>
>
> I see the system svm allocator as just a special case of the driver 
> allocator where not fully backed buffer objects are allocated, but 
> rather sparse one which are filled and migrated on demand.
>
> */[Zeng, Oak] /*
>
> Above statement is true to me. We don’t have BO for system svm 
> allocator. It is a sparse one as we can sparsely map vma to GPU. Our 
> migration policy decide which pages/how much of the vma is 
> migrated/mapped to GPU page table.
>
> *//*
>
> The difference b/t your mind and mine is, you want a gigantic vma 
> (created during the gigantic vm_bind) to be sparsely populated to gpu. 
> While I thought vma (xe_vma in xekmd codes) is a place to save memory 
> attributes (such as caching, user preferred placement etc). All those 
> memory attributes are range based, i.e., user can specify range1 is 
> cached while range2 is uncached. So I don’t see how you can manage it 
> with the gigantic vma. Do you split your gigantic vma later to save 
> range based memory attributes?
>

Yes, exactly that. I mean the splitting and eventually merging of ranges 
is a standard functionality of the GPUVM code.

So when you need to store additional attributes per range then I would 
strongly suggest to make use of this splitting and merging functionality 
as well.

So basically an IOCTL which says range A..B of the GPU address space is 
mapped to offset X of the CPU address space with parameters Y (caching, 
migration behavior etc..). That is essentially the same we have for 
mapping GEM objects, the provider of the backing store is just something 
different.

Regards,
Christian.

> Regards,
>
> Oak
>
>
>
> Regards,
> Christian.
>
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