All of lore.kernel.org
 help / color / mirror / Atom feed
* [PATCH v2] drm/i915/dmabuf: avoid global wbinvd on dma-buf import
@ 2026-08-16  1:45 Prabhakaran, Krishna
  2026-08-16  2:24 ` ✓ i915.CI.BAT: success for " Patchwork
                   ` (4 more replies)
  0 siblings, 5 replies; 11+ messages in thread
From: Prabhakaran, Krishna @ 2026-08-16  1:45 UTC (permalink / raw)
  To: intel-gfx
  Cc: jani.nikula, joonas.lahtinen, rodrigo.vivi, tursulin,
	matthew.auld, thomas.hellstrom, dri-devel, Krishna Prabhakaran

From: Krishna Prabhakaran <krishna.prabhakaran@intel.com>

When i915 needs to make an imported dma-buf coherent for GPU access on
non-LLC platforms, or for objects that bypass LLC, it currently calls
wbinvd_on_all_cpus(). get_pages() runs whenever an imported buffer is
pinned, so this triggers a whole-cache write-back and invalidate,
broadcast by IPI to every CPU, on every execbuf submission involving an
imported dma-buf. That stalls the entire machine for milliseconds and
starves latency-sensitive work on unrelated cores (e.g. USB isochronous
audio serviced on the VMM's main thread).

Flush only the pages that actually need it instead:

 - If we imported one of our own dma-bufs, the backing object is
   struct-page backed once migrated to SMEM, so flush it directly with
   drm_clflush_sg(), exactly as we flush our other objects. This also
   avoids re-entering the exporter through dma_buf_vmap(), which would
   recurse into i915_gem_object_pin_map() on the source object we
   already hold locked (and fails the igt_dmabuf_import_same_driver
   selftest with -EBUSY).

 - For a foreign dma-buf the sg_table is not guaranteed to be backed by
   struct pages, and the importer has no way to tell, so drm_clflush_sg()
   cannot be used. vmap the buffer and flush that virtual range with
   drm_clflush_virt_range() instead: x86 uses PIPT caches, so flushing
   one virtual alias evicts the cache lines for every alias of the same
   physical pages. The dma_resv lock required by dma_buf_vmap() is
   already held here via the imported object.

Fall back to wbinvd only when the buffer cannot be vmapped or is backed
by I/O memory, where there is no CPU-side range to clflush.

Fixes: a035154da45d ("drm/i915/dmabuf: add paranoid flush-on-acquire")
Signed-off-by: Krishna Prabhakaran <krishna.prabhakaran@intel.com>
---
v2:
 - Flush our own imported dma-bufs directly with drm_clflush_sg() instead of
   dma_buf_vmap(), which re-entered i915_gem_object_pin_map() on the source
   object and failed igt_dmabuf_import_same_driver_smem with -EBUSY (reported
   by Intel CI on v1). Foreign dma-bufs still use dma_buf_vmap() +
   drm_clflush_virt_range(); wbinvd only as fallback.
 - Link to v1: https://patchwork.freedesktop.org/patch/744955/?series=171760

 drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c | 55 ++++++++++++++++++----
 1 file changed, 47 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
index b43d34c7d641..c798a90f1c0f 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
@@ -10,6 +10,8 @@
 
 #include <asm/smp.h>
 
+#include <drm/drm_cache.h>
+
 #include "gem/i915_gem_dmabuf.h"
 #include "i915_drv.h"
 #include "i915_gem_object.h"
@@ -249,16 +251,53 @@ static int i915_gem_object_get_pages_dmabuf(struct drm_i915_gem_object *obj)
 	 * DG1 is special here since it still snoops transactions even with
 	 * CACHE_NONE. This is not the case with other HAS_SNOOP platforms. We
 	 * might need to revisit this as we add new discrete platforms.
-	 *
-	 * XXX: Consider doing a vmap flush or something, where possible.
-	 * Currently we just do a heavy handed wbinvd_on_all_cpus() here since
-	 * the underlying sg_table might not even point to struct pages, so we
-	 * can't just call drm_clflush_sg or similar, like we do elsewhere in
-	 * the driver.
 	 */
 	if (i915_gem_object_can_bypass_llc(obj) ||
-	    (!HAS_LLC(i915) && !IS_DG1(i915)))
-		wbinvd_on_all_cpus();
+	    (!HAS_LLC(i915) && !IS_DG1(i915))) {
+		struct dma_buf *dma_buf = obj->base.import_attach->dmabuf;
+
+		if (dma_buf->ops == &i915_dmabuf_ops) {
+			struct drm_i915_gem_object *dma_obj =
+				dma_buf_to_obj(dma_buf);
+
+			/*
+			 * We imported one of our own dma-bufs. The backing
+			 * object is struct-page backed once migrated to SMEM,
+			 * so flush it directly, the same way we flush our
+			 * other objects. This also avoids re-entering the
+			 * exporter through dma_buf_vmap(), which would recurse
+			 * into i915_gem_object_pin_map() on the source object
+			 * we already hold locked.
+			 */
+			if (i915_gem_object_has_struct_page(dma_obj))
+				drm_clflush_sg(dma_obj->mm.pages);
+			else
+				wbinvd_on_all_cpus();
+		} else {
+			struct iosys_map map;
+
+			/*
+			 * A foreign sg_table is not guaranteed to be backed by
+			 * struct pages, so we cannot use drm_clflush_sg(). vmap
+			 * the buffer and flush the virtual range instead; x86
+			 * uses PIPT caches, so flushing one alias evicts the
+			 * lines for every alias of the same physical pages.
+			 *
+			 * We already hold the dma_resv lock via the imported
+			 * obj, so use the locked dma_buf_vmap() variant.
+			 */
+			if (!dma_buf_vmap(dma_buf, &map)) {
+				if (!map.is_iomem)
+					drm_clflush_virt_range(map.vaddr,
+							       obj->base.size);
+				else
+					wbinvd_on_all_cpus();
+				dma_buf_vunmap(dma_buf, &map);
+			} else {
+				wbinvd_on_all_cpus();
+			}
+		}
+	}
 
 	__i915_gem_object_set_pages(obj, sgt);
 

base-commit: 682ea2d28d18bb06f9fc663cb5ab7e80dc0e606a
-- 
2.43.0


^ permalink raw reply related	[flat|nested] 11+ messages in thread
end of thread, other threads:[~2026-08-21  9:09 UTC | newest]

Thread overview: 11+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2026-08-16  1:45 [PATCH v2] drm/i915/dmabuf: avoid global wbinvd on dma-buf import Prabhakaran, Krishna
2026-08-16  2:24 ` ✓ i915.CI.BAT: success for " Patchwork
2026-08-17 20:25 ` ✗ i915.CI.Full: failure " Patchwork
2026-08-20  9:23 ` [PATCH v2] " Matthew Auld
2026-08-20 10:39   ` Thomas Hellström
2026-08-20 20:20     ` Prabhakaran, Krishna
2026-08-20 18:15   ` Prabhakaran, Krishna
2026-08-20 19:09   ` Prabhakaran, Krishna
2026-08-20 20:50 ` [PATCH v3] " Prabhakaran, Krishna
2026-08-21  9:09   ` Matthew Auld
2026-08-20 22:07 ` ✗ i915.CI.BAT: failure for drm/i915/dmabuf: avoid global wbinvd on dma-buf import (rev3) Patchwork

This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.