Re: [PATCH 0/4] drm/gpuvm: Add support for single-page-filled mappings

[Boris Brezillon <boris.brezillon@collabora.com>]

On Mon, 3 Feb 2025 11:23:53 +0000
Liviu Dudau <liviu.dudau@arm.com> wrote:

> On Mon, Feb 03, 2025 at 10:21:53AM +0100, Boris Brezillon wrote:
> > +Akash with whom we've been discussing adding a 'REPEAT' mode to
> > drm_gpuvm/panthor.
> > 
> > On Sun, 2 Feb 2025 19:53:47 +0100
> > Danilo Krummrich <dakr@kernel.org> wrote:
> >   
> > > Hi Lina,
> > > 
> > > On Sun, Feb 02, 2025 at 10:34:49PM +0900, Asahi Lina wrote:  
> > > > Some hardware requires dummy page mappings to efficiently implement
> > > > Vulkan sparse features. These mappings consist of the same physical
> > > > memory page, repeated for a large range of address space (e.g. 16GiB).
> > > > 
> > > > Add support for this to drm_gpuvm. Currently, drm_gpuvm expects BO
> > > > ranges to correspond 1:1 to virtual memory ranges that are mapped, and
> > > > does math on the BO offset accordingly. To make single page mappings
> > > > work, we need a way to turn off that math, keeping the BO offset always
> > > > constant and pointing to the same page (typically BO offset 0).
> > > > 
> > > > To make this work, we need to handle all the corner cases when these
> > > > mappings intersect with regular mappings. The rules are simply to never
> > > > mix or merge a "regular" mapping with a single page mapping.
> > > > 
> > > > drm_gpuvm has support for a flags field in drm_gpuva objects. This is
> > > > normally managed by drivers directly. We can introduce a
> > > > DRM_GPUVA_SINGLE_PAGE flag to handle this. However, to make it work,
> > > > sm_map and friends need to know ahead of time whether the new mapping is
> > > > a single page mapping or not. Therefore, we need to add an argument to
> > > > these functions so drivers can provide the flags to be filled into
> > > > drm_gpuva.flags.
> > > > 
> > > > These changes should not affect any existing drivers that use drm_gpuvm
> > > > other than the API change:
> > > > 
> > > > - imagination: Does not use flags at all
> > > > - nouveau: Only uses drm_gpuva_invalidate(), which is only called on
> > > >   existing drm_gpuva objects (after the map steps)
> > > > - panthor: Does not use flags at all
> > > > - xe: Does not use drm_gpuva_init_from_op() or
> > > >   drm_gpuva_remap()/drm_gpuva_map() (which call it). This means that the
> > > > flags field of the gpuva object is managed by the driver only, so these
> > > > changes cannot clobber it.
> > > > 
> > > > Note that the way this is implemented, drm_gpuvm does not need to know
> > > > the GPU page size. It only has to never do math on the BO offset to meet
> > > > the requirements.
> > > > 
> > > > I suspect that after this change there could be some cleanup possible in
> > > > the xe driver (which right now passes flags around in various
> > > > driver-specific ways from the map step through to drm_gpuva objects),
> > > > but I'll leave that to the Xe folks.
> > > > 
> > > > Signed-off-by: Asahi Lina <lina@asahilina.net>
> > > > ---
> > > > Asahi Lina (4):
> > > >       drm/gpuvm: Add a flags argument to drm_gpuvm_sm_map[_*]
> > > >       drm/gpuvm: Plumb through flags into drm_gpuva_op_map
> > > >       drm/gpuvm: Add DRM_GPUVA_SINGLE_PAGE flag and logic
> > > >       drm/gpuvm: Plumb through flags into drm_gpuva_init    
> > > 
> > > Without looking into any details yet:
> > > 
> > > This is a bit of tricky one, since we're not even close to having a user for
> > > this new feature upstream yet, are we?  
> > 
> > Actually, we would be interesting in having this feature hooked up in
> > panthor. One use case we have is Vulkan sparse bindings, of course. But
> > we also have cases where we need to map a dummy page repeatedly on the
> > FW side. The approach we've been considering is slightly different:
> > pass a DRM_GPUVA_REPEAT_FLAG along with GEM range, so we can repeat a
> > range of the GEM (see the below diff, which is completely untested by
> > the way), but I think we'd be fine with this SINGLE_PAGE flag.  
> 
> Unless I've misunderstood the intent completely, it looks like Xe also wants
> something similar although they call it CPU_ADDR_MIRROR for some reason:
> 
> https://lore.kernel.org/r/20250129195212.745731-9-matthew.brost@intel.com

At first glance, it doesn't seem related. The Xe stuff looks more like
an alloc-on-fault mechanism. SINGLE_PAGE is about mapping a dummy page
repeatedly over a virtual address range so that sparse Vulkan images
never get a fault when an unbound image region is accessed.