On Tue, Jul 13, 2021 at 02:04:31PM +0100, Matthew Auld wrote:
We skip filling out the pt with scratch entries if the va range covers the entire pt, since we later have to fill it with the PTEs for the object pages anyway. However this might leave open a small window where the PTEs don't point to anything valid for the HW to consume.
When for example using 2M GTT pages this fill_px() showed up as being quite significant in perf measurements, and ends up being completely wasted since we ignore the pt and just use the pde directly.
Anyway, currently we have our PTE construction split between alloc and insert, which is probably slightly iffy nowadays, since the alloc doesn't actually allocate anything anymore, instead it just sets up the page directories and points the PTEs at the scratch page. Later when we do the insert step we re-program the PTEs again. Better might be to squash the alloc and insert into a single step, then bringing back this optimisation(along with some others) should be possible.
Fixes: 14826673247e ("drm/i915: Only initialize partially filled pagetables") Signed-off-by: Matthew Auld matthew.auld@intel.com Cc: Jon Bloomfield jon.bloomfield@intel.com Cc: Chris Wilson chris.p.wilson@intel.com Cc: Daniel Vetter daniel@ffwll.ch Cc: stable@vger.kernel.org # v4.15+
This is some impressively convoluted code, and I'm scared.
But as far as I managed to convince myself, your story here checks out. Problem will be a bit that this code moved around a _lot_ so we'll need a lot of dedicated backports :-(
Reviewed-by: Daniel Vetter daniel.vetter@ffwll.ch
drivers/gpu/drm/i915/gt/gen8_ppgtt.c | 5 +---- 1 file changed, 1 insertion(+), 4 deletions(-)
diff --git a/drivers/gpu/drm/i915/gt/gen8_ppgtt.c b/drivers/gpu/drm/i915/gt/gen8_ppgtt.c index 3d02c726c746..6e0e52eeb87a 100644 --- a/drivers/gpu/drm/i915/gt/gen8_ppgtt.c +++ b/drivers/gpu/drm/i915/gt/gen8_ppgtt.c @@ -303,10 +303,7 @@ static void __gen8_ppgtt_alloc(struct i915_address_space * const vm, __i915_gem_object_pin_pages(pt->base); i915_gem_object_make_unshrinkable(pt->base);
if (lvl ||gen8_pt_count(*start, end) < I915_PDES ||intel_vgpu_active(vm->i915))fill_px(pt, vm->scratch[lvl]->encode);
fill_px(pt, vm->scratch[lvl]->encode);spin_lock(&pd->lock); if (likely(!pd->entry[idx])) { -- 2.26.3