[RFC PATCH 0/6] Supporting GMEM (generalized memory management) for external memory devices

[Dave Airlie]

On Tue, 28 Nov 2023 at 23:07, Christian König <christian.koenig at amd.com> wrote:
>
> Am 28.11.23 um 13:50 schrieb Weixi Zhu:
> > The problem:
> >
> > Accelerator driver developers are forced to reinvent external MM subsystems
> > case by case, because Linux core MM only considers host memory resources.
> > These reinvented MM subsystems have similar orders of magnitude of LoC as
> > Linux MM (80K), e.g. Nvidia-UVM has 70K, AMD GPU has 14K and Huawei NPU has
> > 30K. Meanwhile, more and more vendors are implementing their own
> > accelerators, e.g. Microsoft's Maia 100. At the same time,
> > application-level developers suffer from poor programmability -- they must
> > consider parallel address spaces and be careful about the limited device
> > DRAM capacity. This can be alleviated if a malloc()-ed virtual address can
> > be shared by the accelerator, or the abundant host DRAM can further
> > transparently backup the device local memory.
> >
> > These external MM systems share similar mechanisms except for the
> > hardware-dependent part, so reinventing them is effectively introducing
> > redundant code (14K~70K for each case). Such developing/maintaining is not
> > cheap. Furthermore, to share a malloc()-ed virtual address, device drivers
> > need to deeply interact with Linux MM via low-level MM APIs, e.g. MMU
> > notifiers/HMM. This raises the bar for driver development, since developers
> > must understand how Linux MM works. Further, it creates code maintenance
> > problems -- any changes to Linux MM potentially require coordinated changes
> > to accelerator drivers using low-level MM APIs.
> >
> > Putting a cache-coherent bus between host and device will not make these
> > external MM subsystems disappear. For example, a throughput-oriented
> > accelerator will not tolerate executing heavy memory access workload with
> > a host MMU/IOMMU via a remote bus. Therefore, devices will still have
> > their own MMU and pick a simpler page table format for lower address
> > translation overhead, requiring external MM subsystems.
> >
> > --------------------
> >
> > What GMEM (Generalized Memory Management [1]) does:
> >
> > GMEM extends Linux MM to share its machine-independent MM code. Only
> > high-level interface is provided for device drivers. This prevents
> > accelerator drivers from reinventing the wheel, but relies on drivers to
> > implement their hardware-dependent functions declared by GMEM. GMEM's key
> > interface include gm_dev_create(), gm_as_create(), gm_as_attach() and
> > gm_dev_register_physmem(). Here briefly describe how a device driver
> > utilizes them:
> > 1. At boot time, call gm_dev_create() and registers the implementation of
> >     hardware-dependent functions as declared in struct gm_mmu.
> >       - If the device has local DRAM, call gm_dev_register_physmem() to
> >         register available physical addresses.
> > 2. When a device context is initialized (e.g. triggered by ioctl), check if
> >     the current CPU process has been attached to a gmem address space
> >     (struct gm_as). If not, call gm_as_create() and point current->mm->gm_as
> >     to it.
> > 3. Call gm_as_attach() to attach the device context to a gmem address space.
> > 4. Invoke gm_dev_fault() to resolve a page fault or prepare data before
> >     device computation happens.
> >
> > GMEM has changed the following assumptions in Linux MM:
> >    1. An mm_struct not only handle a single CPU context, but may also handle
> >       external memory contexts encapsulated as gm_context listed in
> >       mm->gm_as. An external memory context can include a few or all of the
> >       following parts: an external MMU (that requires TLB invalidation), an
> >       external page table (that requires PTE manipulation) and external DRAM
> >       (that requires physical memory management).
>
> Well that is pretty much exactly what AMD has already proposed with KFD
> and was rejected for rather good reasons.

> >
> > MMU functions
> > The MMU functions peer_map() and peer_unmap() overlap other functions,
> > leaving a question if the MMU functions should be decoupled as more basic
> > operations. Decoupling them could potentially prevent device drivers
> > coalescing these basic steps within a single host-device communication
> > operation, while coupling them makes it more difficult for device drivers
> > to utilize GMEM interface.
>
> Well to be honest all of this sounds like history to me. We have already
> seen the same basic approach in KFD, HMM and to some extend in TTM as well.
>
> And all of them more or less failed. Why should this here be different?


Any info we have on why this has failed to work in the past would be
useful to provide. This is one of those cases where we may not have
documented the bad ideas to stop future developers from thinking they
are bad.

I do think we would want more common code in this area, but I would
think we'd have it more on the driver infrastructure side, than in the
core mm.

Dave.