On 2024-01-29 14:03, Christian König wrote:
Am 29.01.24 um 18:52 schrieb Felix Kuehling:
On 2024-01-29 11:28, Christian König wrote:
Am 29.01.24 um 17:24 schrieb Felix Kuehling:
On 2024-01-29 10:33, Christian König wrote:
Am 29.01.24 um 16:03 schrieb Felix Kuehling:
On 2024-01-25 13:32, Daniel Vetter wrote:
On Wed, Jan 24, 2024 at 09:33:12AM +0100, Christian König wrote:
Am 23.01.24 um 20:37 schrieb Zeng, Oak:
[SNIP]
Yes most API are per device based.
One exception I know is actually the kfd SVM API. If you look
at the svm_ioctl function, it is per-process based. Each
kfd_process represent a process across N gpu devices.
Yeah and that was a big mistake in my opinion. We should really
not do that
ever again.
Need to say, kfd SVM represent a shared virtual address space
across CPU and all GPU devices on the system. This is by the
definition of SVM (shared virtual memory). This is very
different from our legacy gpu *device* driver which works for
only one device (i.e., if you want one device to access
another device's memory, you will have to use dma-buf
export/import etc).
Exactly that thinking is what we have currently found as
blocker for a
virtualization projects. Having SVM as device independent
feature which
somehow ties to the process address space turned out to be an
extremely bad
idea.
The background is that this only works for some use cases but
not all of
them.
What's working much better is to just have a mirror
functionality which says
that a range A..B of the process address space is mapped into a
range C..D
of the GPU address space.
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.
When you talk about migrating memory to a device you also do
this on a per
device basis and *not* tied to the process address space. If
you then get
crappy performance because userspace gave contradicting
information where to
migrate memory then that's a bug in userspace and not something
the kernel
should try to prevent somehow.
[SNIP]
I think if you start using the same drm_gpuvm for multiple
devices you
will sooner or later start to run into the same mess we have
seen with
KFD, where we moved more and more functionality from the KFD
to the DRM
render node because we found that a lot of the stuff simply
doesn't work
correctly with a single object to maintain the state.
As I understand it, KFD is designed to work across devices. A
single pseudo /dev/kfd device represent all hardware gpu
devices. That is why during kfd open, many pdd (process device
data) is created, each for one hardware device for this process.
Yes, I'm perfectly aware of that. And I can only repeat myself
that I see
this design as a rather extreme failure. And I think it's one
of the reasons
why NVidia is so dominant with Cuda.
This whole approach KFD takes was designed with the idea of
extending the
CPU process into the GPUs, but this idea only works for a few
use cases and
is not something we should apply to drivers in general.
A very good example are virtualization use cases where you end
up with CPU
address != GPU address because the VAs are actually coming from
the guest VM
and not the host process.
SVM is a high level concept of OpenCL, Cuda, ROCm etc.. This
should not have
any influence on the design of the kernel UAPI.
If you want to do something similar as KFD for Xe I think you
need to get
explicit permission to do this from Dave and Daniel and maybe
even Linus.
I think the one and only one exception where an SVM uapi like in
kfd makes
sense, is if the _hardware_ itself, not the software stack defined
semantics that you've happened to build on top of that hw,
enforces a 1:1
mapping with the cpu process address space.
Which means your hardware is using PASID, IOMMU based
translation, PCI-ATS
(address translation services) or whatever your hw calls it and
has _no_
device-side pagetables on top. Which from what I've seen all
devices with
device-memory have, simply because they need some place to store
whether
that memory is currently in device memory or should be
translated using
PASID. Currently there's no gpu that works with PASID only, but
there are
some on-cpu-die accelerator things that do work like that.
Maybe in the future there will be some accelerators that are
fully cpu
cache coherent (including atomics) with something like CXL, and the
on-device memory is managed as normal system memory with struct
page as
ZONE_DEVICE and accelerator va -> physical address translation
is only
done with PASID ... but for now I haven't seen that, definitely
not in
upstream drivers.
And the moment you have some per-device pagetables or per-device
memory
management of some sort (like using gpuva mgr) then I'm 100%
agreeing with
Christian that the kfd SVM model is too strict and not a great
idea.
That basically means, without ATS/PRI+PASID you cannot implement
a unified memory programming model, where GPUs or accelerators
access virtual addresses without pre-registering them with an SVM
API call.
Unified memory is a feature implemented by the KFD SVM API and
used by ROCm. This is used e.g. to implement OpenMP USM (unified
shared memory). It's implemented with recoverable GPU page
faults. If the page fault interrupt handler cannot assume a
shared virtual address space, then implementing this feature
isn't possible.
Why not? As far as I can see the OpenMP USM is just another funky
way of userptr handling.
The difference is that in an userptr we assume that we always need
to request the whole block A..B from a mapping while for page
fault based handling it can be just any page in between A and B
which is requested and made available to the GPU address space.
As far as I can see there is absolutely no need for any special
SVM handling.
It does assume a shared virtual address space between CPU and GPUs.
There are no API calls to tell the driver that address A on the CPU
maps to address B on the GPU1 and address C on GPU2. The KFD SVM
API was designed to work with this programming model, by augmenting
the shared virtual address mappings with virtual address range
attributes that can modify the migration policy and indicate
prefetching, prefaulting, etc. You could think of it as madvise on
steroids.
Yeah, so what? In this case you just say through an IOCTL that CPU
range A..B should map to GPU range C..D and for A/B and C/D you use
the maximum of the address space.
What I want is that address range A..B on the CPU matches A..B on the
GPU, because I'm sharing pointers between CPU and GPU. I can't think
of any sane user mode using a unified memory programming model, that
would ever ask KFD to map unified memory mappints to a different
address range on the GPU. Adding such an ioclt is a complete waste of
time, and can only serve to add unnecessary complexity.
This is exactly the use case which happens when the submitting process
is not the one originally stitching together the command stream.
Basically all native context, virtualization and other proxy use cases
work like this.
You cannot use unified memory in this type of environment. A GPU page
fault would occur in a GPU virtual address space in the host-side proxy
process. That page fault would need to be resolved to map some memory
from a process running in a guest? Which guest? Which process? That's
anyone's guess. There is no way to annotate that because the pointer in
the fault is coming from a shader program that's running in the guest
context and competely unaware of the virtualization. There are no API
calls from the guest before the fault occurs to establish a meaningful
mapping.
The way this virtualization of the GPU is implemented, with out proxy
multiplexing multiple clients is just fundamentally incompatible with a
unified memory programming model that has to assume a shared virtual
address space to make sense. You'd need separate proxy processes on the
host side to handle that. You can't blame this on bad design decisions
in the SVM API. As I see it, it's just a fundamental limitation of the
virtualization approach that cannot handle guest applications that want
to use a unified shared memory programming model.
That's why I suggested to completely disable the SVM API in this
virtualization scenario when you create a KFD context that's separate
from the process address space. It is not essential for any
non-unified-memory functionality. ROCm user mode has fallbacks to work
without it, because we also needs to support older kernels and GPUs that
didn't support this programming model.
Regards,
Felix
So that the CPU address doesn't match the GPU address is an absolutely
real use case and should be able to be handled by the GPU VA interface.
Regards,
Christian.
Regards,
Felix
There is no restriction that this needs to be accurate in way. It's
just the it can be accurate to be more efficient and eventually use
only a fraction of the address space instead of all of it for some
use cases.
So this isn't a blocker, it's just one special use case.
Regards,
Christian.
Regards,
Felix
Regards,
Christian.
Regards,
Felix
Cheers, Sima
