Hi Andrzej, On Mon, Feb 06, 2023 at 10:03:27AM +0100, Andrzej Hajda wrote: > Write-combining memory allows speculative reads by CPU. > ggtt->error_capture is WC mapped to CPU, so CPU/MMU can try > to prefetch memory beyond the error_capture, ie it tries > to read memory pointed by next PTE in GGTT. > If this PTE points to invalid address DMAR errors will occur. > This behaviour was observed on ADL, RPL, DG2 platforms. > To avoid it, guard scratch page should be added after error_capture. > > Signed-off-by: Andrzej Hajda <andrzej.hajda@xxxxxxxxx> Reviewed-by: Andi Shyti <andi.shyti@xxxxxxxxxxxxxxx> Thanks, Andi > --- > This patch tries to diminish plague of DMAR read errors present > in CI for ADL*, RPL*, DG2 platforms, see for example [1] (grep DMAR). > CI is usually tolerant for these errors, so the scale of the problem > is not really visible. > To show it I have counted lines containing DMAR read errors in dmesgs > produced by CI for all three versions of the patch, but in contrast to v2 > I have grepped only for lines containing "PTE Read access". > Below stats for kernel w/o patch vs patched one. > v1: 210 vs 0 > v2: 201 vs 0 > v3: 214 vs 0 > Apparently the patch fixes all common PTE read errors. > > In previous version there were different numbers due to less exact grepping, > "grep DMAR" catched write errors and "DMAR: DRHD: handling fault status reg" > lines, anyway the actual number of errors is much bigger - DMAR errors > are rate-limited. > > [1]: http://gfx-ci.igk.intel.com/tree/drm-tip/CI_DRM_12678/bat-adln-1/dmesg0.txt > > Changelog: > v2: > - modified commit message (I hope the diagnosis is correct), > - added bug checks to ensure scratch is initialized on gen3 platforms. > CI produces strange stacktrace for it suggesting scratch[0] is NULL, > to be removed after resolving the issue with gen3 platforms. > v3: > - removed bug checks, replaced with gen check. > > Regards > Andrzej > --- > drivers/gpu/drm/i915/gt/intel_ggtt.c | 30 +++++++++++++++++++++++++--- > 1 file changed, 27 insertions(+), 3 deletions(-) > > diff --git a/drivers/gpu/drm/i915/gt/intel_ggtt.c b/drivers/gpu/drm/i915/gt/intel_ggtt.c > index 842e69c7b21e49..15eb2d4158d679 100644 > --- a/drivers/gpu/drm/i915/gt/intel_ggtt.c > +++ b/drivers/gpu/drm/i915/gt/intel_ggtt.c > @@ -503,6 +503,13 @@ static void cleanup_init_ggtt(struct i915_ggtt *ggtt) > mutex_destroy(&ggtt->error_mutex); > } > > +static void ggtt_insert_scratch_page(struct i915_ggtt *ggtt, u64 offset) > +{ > + struct i915_address_space *vm = &ggtt->vm; > + > + vm->insert_page(vm, px_dma(vm->scratch[0]), offset, I915_CACHE_NONE, 0); > +} > + > static int init_ggtt(struct i915_ggtt *ggtt) > { > /* > @@ -553,6 +560,13 @@ static int init_ggtt(struct i915_ggtt *ggtt) > * bug, which we expect to cause other failures... > */ > ggtt->error_capture.size = I915_GTT_PAGE_SIZE; > + /* > + * Since CPU can perform speculative reads on error capture > + * (write-combining allows it) and since gen12 it really happens > + * add scratch page after error capture to avoid DMAR errors. > + */ > + if (GRAPHICS_VER(ggtt->vm.i915) >= 12) > + ggtt->error_capture.size += I915_GTT_PAGE_SIZE; > ggtt->error_capture.color = I915_COLOR_UNEVICTABLE; > if (drm_mm_reserve_node(&ggtt->vm.mm, &ggtt->error_capture)) > drm_mm_insert_node_in_range(&ggtt->vm.mm, > @@ -562,11 +576,21 @@ static int init_ggtt(struct i915_ggtt *ggtt) > 0, ggtt->mappable_end, > DRM_MM_INSERT_LOW); > } > - if (drm_mm_node_allocated(&ggtt->error_capture)) > + if (drm_mm_node_allocated(&ggtt->error_capture)) { > + u64 start = ggtt->error_capture.start; > + u64 end = ggtt->error_capture.start + ggtt->error_capture.size; > + u64 i; > + > + /* > + * During error capture, memcpying from the GGTT is triggering a > + * prefetch of the following PTE, so fill it with a guard page. > + */ > + for (i = start + I915_GTT_PAGE_SIZE; i < end; i += I915_GTT_PAGE_SIZE) > + ggtt_insert_scratch_page(ggtt, i); > drm_dbg(&ggtt->vm.i915->drm, > "Reserved GGTT:[%llx, %llx] for use by error capture\n", > - ggtt->error_capture.start, > - ggtt->error_capture.start + ggtt->error_capture.size); > + start, end); > + } > > /* > * The upper portion of the GuC address space has a sizeable hole > -- > 2.34.1
