Am 28.02.24 um 20:51 schrieb Zeng, Oak:
The mail wasn’t indent/preface correctly. Manually format it.
*From:*Christian König <christian.koe...@amd.com>
*Sent:* Tuesday, February 27, 2024 1:54 AM
*To:* Zeng, Oak <oak.z...@intel.com>; Danilo Krummrich
<d...@redhat.com>; Dave Airlie <airl...@redhat.com>; Daniel Vetter
<dan...@ffwll.ch>; Felix Kuehling <felix.kuehl...@amd.com>;
jgli...@redhat.com
*Cc:* Welty, Brian <brian.we...@intel.com>;
dri-devel@lists.freedesktop.org; intel...@lists.freedesktop.org;
Bommu, Krishnaiah <krishnaiah.bo...@intel.com>; Ghimiray, Himal Prasad
<himal.prasad.ghimi...@intel.com>; thomas.hellst...@linux.intel.com;
Vishwanathapura, Niranjana <niranjana.vishwanathap...@intel.com>;
Brost, Matthew <matthew.br...@intel.com>; Gupta, saurabhg
<saurabhg.gu...@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.
