[PATCH v4] drm/i915/dmabuf: avoid global wbinvd on dma-buf import

["Prabhakaran, Krishna" <krishna.prabhakaran@intel.com>]

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
   normally 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). SMEM does not
   guarantee the struct-page flag though: phys objects clear it and it
   is dropped transiently during TTM moves, and drm_clflush_sg() walks
   the sgt with sg_page(), so guard the direct flush with
   i915_gem_object_has_struct_page() and fall back to wbinvd when it is
   not set.

 - 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>
---
v4: Guard the own-dma-buf direct drm_clflush_sg() with
    i915_gem_object_has_struct_page() and fall back to wbinvd when the
    SMEM object is not struct-page backed (phys objects, transient TTM
    moves); sg_page() would otherwise walk bogus pages. (review feedback)
v3: Drop the wbinvd fallback inside the own-dma-buf branch; the exporter is
    migrated to SMEM on attach, so flushing device memory made no sense.
    (Matthew Auld)
v2: Special-case our own dma-bufs with drm_clflush_sg() instead of
    dma_buf_vmap(), which recursed into i915_gem_object_pin_map() on the
    already-locked source object and failed igt_dmabuf_import_same_driver
    with -EBUSY; foreign dma-bufs use vmap + drm_clflush_virt_range().
v1: https://patchwork.freedesktop.org/series/171760/
 drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c | 62 +++++++++++++++++++---
 1 file changed, 54 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..d1695171019c 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 +251,60 @@ 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 exporter is
+			 * migrated to SMEM on attach, so it is normally struct-page
+			 * backed and we can 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.
+			 *
+			 * SMEM does not guarantee the STRUCT_PAGE flag though: phys
+			 * objects clear it and it is dropped transiently during TTM
+			 * moves. drm_clflush_sg() walks the sgt with sg_page(), so
+			 * without struct pages those pointers are bogus; fall back to
+			 * wbinvd in that case (not the vmap path, which would recurse
+			 * on the already-locked source object).
+			 */
+			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);
 
-- 
2.43.0