Implement svm without BO concept in xe driver

[Zeng, Oak]

> -----Original Message-----
> From: dri-devel <dri-devel-bounces at lists.freedesktop.org> On Behalf Of Dave
> Airlie
> Sent: August 20, 2023 6:21 PM
> To: Zeng, Oak <oak.zeng at intel.com>
> Cc: Brost, Matthew <matthew.brost at intel.com>; Thomas Hellström
> <thomas.hellstrom at linux.intel.com>; Philip Yang <Philip.Yang at amd.com>; Felix
> Kuehling <felix.kuehling at amd.com>; Welty, Brian <brian.welty at intel.com>; dri-
> devel at lists.freedesktop.org; intel-xe at lists.freedesktop.org; Vishwanathapura,
> Niranjana <niranjana.vishwanathapura at intel.com>; Christian König
> <christian.koenig at amd.com>
> Subject: Re: Implement svm without BO concept in xe driver
> 
> On Thu, 17 Aug 2023 at 12:13, Zeng, Oak <oak.zeng at intel.com> wrote:
> >
> > > -----Original Message-----
> > > From: Dave Airlie <airlied at gmail.com>
> > > Sent: August 16, 2023 6:52 PM
> > > To: Felix Kuehling <felix.kuehling at amd.com>
> > > Cc: Zeng, Oak <oak.zeng at intel.com>; Christian König
> > > <christian.koenig at amd.com>; Thomas Hellström
> > > <thomas.hellstrom at linux.intel.com>; Brost, Matthew
> > > <matthew.brost at intel.com>; maarten.lankhorst at linux.intel.com;
> > > Vishwanathapura, Niranjana <niranjana.vishwanathapura at intel.com>; Welty,
> > > Brian <brian.welty at intel.com>; Philip Yang <Philip.Yang at amd.com>; intel-
> > > xe at lists.freedesktop.org; dri-devel at lists.freedesktop.org
> > > Subject: Re: Implement svm without BO concept in xe driver
> > >
> > > On Thu, 17 Aug 2023 at 08:15, Felix Kuehling <felix.kuehling at amd.com>
> wrote:
> > > >
> > > > On 2023-08-16 13:30, Zeng, Oak wrote:
> > > > > I spoke with Thomas. We discussed two approaches:
> > > > >
> > > > > 1) make ttm_resource a central place for vram management functions
> such as
> > > eviction, cgroup memory accounting. Both the BO-based driver and BO-less
> SVM
> > > codes call into ttm_resource_alloc/free functions for vram allocation/free.
> > > > >      *This way BO driver and SVM driver shares the eviction/cgroup logic, no
> > > need to reimplment LRU eviction list in SVM driver. Cgroup logic should be in
> > > ttm_resource layer. +Maarten.
> > > > >      *ttm_resource is not a perfect match for SVM to allocate vram. It is still
> a
> > > big overhead. The *bo* member of ttm_resource is not needed for SVM -
> this
> > > might end up with invasive changes to ttm...need to look into more details
> > > >
> > > > Overhead is a problem. We'd want to be able to allocate, free and evict
> > > > memory at a similar granularity as our preferred migration and page
> > > > fault granularity, which defaults to 2MB in our SVM implementation.
> > > >
> > > >
> > > > >
> > > > > 2) svm code allocate memory directly from drm-buddy allocator, and
> expose
> > > memory eviction functions from both ttm and svm so they can evict memory
> > > from each other. For example, expose the ttm_mem_evict_first function
> from
> > > ttm side so hmm/svm code can call it; expose a similar function from svm side
> so
> > > ttm can evict hmm memory.
> > > >
> > > > I like this option. One thing that needs some thought with this is how
> > > > to get some semblance of fairness between the two types of clients.
> > > > Basically how to choose what to evict. And what share of the available
> > > > memory does each side get to use on average. E.g. an idle client may get
> > > > all its memory evicted while a busy client may get a bigger share of the
> > > > available memory.
> > >
> > > I'd also like to suggest we try to write any management/generic code
> > > in driver agnostic way as much as possible here. I don't really see
> > > much hw difference should be influencing it.
> > >
> > > I do worry about having effectively 2 LRUs here, you can't really have
> > > two "leasts".
> > >
> > > Like if we hit the shrinker paths who goes first? do we shrink one
> > > object from each side in turn?
> >
> > One way to solve this fairness problem is to create a driver agnostic
> drm_vram_mgr. Maintain a single LRU in drm_vram_mgr. Move the memory
> eviction/cgroups memory accounting logic from ttm_resource manager to
> drm_vram_mgr. Both BO-based driver and SVM driver calls to drm_vram_mgr to
> allocate/free memory.
> >
> > I am not sure whether this meets the 2M allocate/free/evict granularity
> requirement Felix mentioned above. SVM can allocate 2M size blocks. But BO
> driver should be able to allocate any arbitrary sized blocks - So the eviction is also
> arbitrary size.
> >
> > >
> > > Also will we have systems where we can expose system SVM but userspace
> > > may choose to not use the fine grained SVM and use one of the older
> > > modes, will that path get emulated on top of SVM or use the BO paths?
> >
> >
> > If by "older modes" you meant the gem_bo_create (such as xe_gem_create or
> amdgpu_gem_create), then today both amd and intel implement those
> interfaces using BO path. We don't have a plan to emulate that old mode on tope
> of SVM, afaict.
> 
> I'm not sure how the older modes manifest in the kernel I assume as bo
> creates (but they may use userptr), SVM isn't a specific thing, it's a
> group of 3 things.
> 
> 1) coarse-grained SVM which I think is BO
> 2) fine-grained SVM which is page level
> 3) fine-grained system SVM which is HMM
> 
> I suppose I'm asking about the previous versions and how they would
> operate in a system SVM capable system.

I got your question now. 

As I understand it, the system SVM provides similar functionality as BO-based SVM (i.e., share virtual address space b/t cpu and gpu program, no explicit memory placement for gpu program), but they have different user interface (malloc, mmap vs bo create, vm bind).

From functionality perspective, on a system SVM capable system, we don't need #1/#2. Once #3 is implemented and turned out be as performant as #1/#2, we can ask user space to switch to #3.

As far as I know, AMD doesn't have #1/#2 - their BO-based driver *requires* all valid GPU virtual address be mapped to GPU page table *before* GPU kernel submission, aka a GPU page fault is treated as fatal. Felix please fix me, as my AMD knowledge is fading away...

From interface perspective, i.e., to keep UMD which using #1/#2 continue to run without modification, we need #1/#2 to continue exist.

Should we emulate #1/#2 on top of #3? I feel the BO-based memory management and the struct page/hmm based memory management are quite different design philosophy. Trying to emulate one on top of another can run into serious difficulty. For example, how do we emulate a vm_bind on top of #3? Remember for #1/#2 virtual address space is managed by user space while #3 virtual address space is managed by kernel core mm (vma struct...). It is a hard conflict here...

Thanks again for the great question!
Oak 

> 
> Dave.
> >
> > Thanks,
> > Oak
> >
> > >
> > > Dave.