Re: [PATCH v8 5/5] drm/i915/uapi: document behaviour for DG2 64K support

[Ramalingam C]

On 2022-02-18 at 18:06:00 +0000, Robert Beckett wrote:
> 
> 
> On 18/02/2022 13:47, Ramalingam C wrote:
> > On 2022-02-17 at 20:57:35 -0800, Jordan Justen wrote:
> > > Robert Beckett <bob.beckett@xxxxxxxxxxxxx> writes:
> > > 
> > > > From: Matthew Auld <matthew.auld@xxxxxxxxx>
> > > > 
> > > > On discrete platforms like DG2, we need to support a minimum page size
> > > > of 64K when dealing with device local-memory. This is quite tricky for
> > > > various reasons, so try to document the new implicit uapi for this.
> > > > 
> > > > v3: fix typos and less emphasis
> > > > v2: Fixed suggestions on formatting [Daniel]
> > > > 
> > > > Signed-off-by: Matthew Auld <matthew.auld@xxxxxxxxx>
> > > > Signed-off-by: Ramalingam C <ramalingam.c@xxxxxxxxx>
> > > > Signed-off-by: Robert Beckett <bob.beckett@xxxxxxxxxxxxx>
> > > > Acked-by: Jordan Justen <jordan.l.justen@xxxxxxxxx>
> > > > Reviewed-by: Ramalingam C <ramalingam.c@xxxxxxxxx>
> > > > Reviewed-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
> > > > cc: Simon Ser <contact@xxxxxxxxxxx>
> > > > cc: Pekka Paalanen <ppaalanen@xxxxxxxxx>
> > > > Cc: Jordan Justen <jordan.l.justen@xxxxxxxxx>
> > > > Cc: Kenneth Graunke <kenneth@xxxxxxxxxxxxx>
> > > > Cc: mesa-dev@xxxxxxxxxxxxxxxxxxxxx
> > > > Cc: Tony Ye <tony.ye@xxxxxxxxx>
> > > > Cc: Slawomir Milczarek <slawomir.milczarek@xxxxxxxxx>
> > > > ---
> > > >   include/uapi/drm/i915_drm.h | 44 ++++++++++++++++++++++++++++++++-----
> > > >   1 file changed, 39 insertions(+), 5 deletions(-)
> > > > 
> > > > diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
> > > > index 5e678917da70..77e5e74c32c1 100644
> > > > --- a/include/uapi/drm/i915_drm.h
> > > > +++ b/include/uapi/drm/i915_drm.h
> > > > @@ -1118,10 +1118,16 @@ struct drm_i915_gem_exec_object2 {
> > > >   	/**
> > > >   	 * When the EXEC_OBJECT_PINNED flag is specified this is populated by
> > > >   	 * the user with the GTT offset at which this object will be pinned.
> > > > +	 *
> > > >   	 * When the I915_EXEC_NO_RELOC flag is specified this must contain the
> > > >   	 * presumed_offset of the object.
> > > > +	 *
> > > >   	 * During execbuffer2 the kernel populates it with the value of the
> > > >   	 * current GTT offset of the object, for future presumed_offset writes.
> > > > +	 *
> > > > +	 * See struct drm_i915_gem_create_ext for the rules when dealing with
> > > > +	 * alignment restrictions with I915_MEMORY_CLASS_DEVICE, on devices with
> > > > +	 * minimum page sizes, like DG2.
> > > >   	 */
> > > >   	__u64 offset;
> > > > @@ -3145,11 +3151,39 @@ struct drm_i915_gem_create_ext {
> > > >   	 *
> > > >   	 * The (page-aligned) allocated size for the object will be returned.
> > > >   	 *
> > > > -	 * Note that for some devices we have might have further minimum
> > > > -	 * page-size restrictions(larger than 4K), like for device local-memory.
> > > > -	 * However in general the final size here should always reflect any
> > > > -	 * rounding up, if for example using the I915_GEM_CREATE_EXT_MEMORY_REGIONS
> > > > -	 * extension to place the object in device local-memory.
> > > > +	 *
> > > > +	 * DG2 64K min page size implications:
> > > > +	 *
> > > > +	 * On discrete platforms, starting from DG2, we have to contend with GTT
> > > > +	 * page size restrictions when dealing with I915_MEMORY_CLASS_DEVICE
> > > > +	 * objects.  Specifically the hardware only supports 64K or larger GTT
> > > > +	 * page sizes for such memory. The kernel will already ensure that all
> > > > +	 * I915_MEMORY_CLASS_DEVICE memory is allocated using 64K or larger page
> > > > +	 * sizes underneath.
> > > > +	 *
> > > > +	 * Note that the returned size here will always reflect any required
> > > > +	 * rounding up done by the kernel, i.e 4K will now become 64K on devices
> > > > +	 * such as DG2.
> > > > +	 *
> > > > +	 * Special DG2 GTT address alignment requirement:
> > > > +	 *
> > > > +	 * The GTT alignment will also need to be at least 2M for such objects.
> > > > +	 *
> > > > +	 * Note that due to how the hardware implements 64K GTT page support, we
> > > > +	 * have some further complications:
> > > > +	 *
> > > > +	 *   1) The entire PDE (which covers a 2MB virtual address range), must
> > > > +	 *   contain only 64K PTEs, i.e mixing 4K and 64K PTEs in the same
> > > > +	 *   PDE is forbidden by the hardware.
> > > > +	 *
> > > > +	 *   2) We still need to support 4K PTEs for I915_MEMORY_CLASS_SYSTEM
> > > > +	 *   objects.
> > > > +	 *
> > > > +	 * To keep things simple for userland, we mandate that any GTT mappings
> > > > +	 * must be aligned to and rounded up to 2MB.
> > > 
> > > Could I get a clarification about this "rounded up" part.
> > > 
> > > Currently Mesa is aligning the start of each and every buffer VMA to be
> > > 2MiB aligned. But, we are *not* taking any steps to "round up" the size
> > > of buffers to 2MiB alignment.
> > > 
> > > Bob's Mesa MR from a while ago,
> > > 
> > > https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14599
> > > 
> > > was trying to add this "round up" size for buffers. We didn't accept
> > > this MR because we thought if we have ensured that no other buffer will
> > > use the same 2MiB VMA range, then it should not be required.
> > > 
> > > If what we are doing is ok, then maybe this "round up" language should
> > > be dropped? Or, perhaps the "round up" mentioned here isn't implying we
> > > must align the size of buffers that we create, and I'm misinterpreting
> > > this.
> > Jordan,
> > 
> > as per my understanding this size rounding up to 2MB is for the VMA mapping,
> > not for the buffer size.
> correct, only the vma is rounded up
> 
> > 
> > Even if we drop this rounding up of vma size to 2MB but align the VMA
> > start to 2MB address then also this should be fine. Becasue the remaining of the
> > last PDE(2MB) will never be used for any other GTT mapping as the
> > starting addr wont align to 2MB.
> The kernel has to handle 4K pages also, which could in theory attempt to be
> placed in any remaining space within a 2MB region, which is not supported.
> For this reason, the kernel rounds up the vma to next 2MB to ensure no 4K
> pages can treat the remaining space as a candidate for placement.
> 
> For mesa, this is not required as they only ever use 2MB alignment for all
> buffers, hence the denial of the mesa mr.
> 
> Internally, the kernel will still round up the vma reservations to prevent
> any kernel 4K buffers being situated in remaining space.
> 
> If desired, we can make the wording clearer, maybe something like:
> 
> "To keep things simple for userland, we mandate that any GTT mappings
> must be aligned to 2MB. The kernel will internally pad them out to the next
> 2MB boundary"

Added the extra information in next version @ https://patchwork.freedesktop.org/patch/475166/?series=100419&rev=1

Jordan, hope this explanation clears your doubt.

Ram.
> 
> 
> > 
> > Bob, Is the above understanding is right? if so could we drop the
> > requirement of mapping the vma size to 2MB?
> > 
> > Ram
> > > 
> > > -Jordan
> > > 
> > > > As this only wastes virtual
> > > > +	 * address space and avoids userland having to copy any needlessly
> > > > +	 * complicated PDE sharing scheme (coloring) and only affects DG2, this
> > > > +	 * is deemed to be a good compromise.
> > > >   	 */
> > > >   	__u64 size;
> > > >   	/**
> > > > -- 
> > > > 2.25.1