On 02/09/2026 20:31, Prabhakaran, Krishna wrote:
From: Krishna Prabhakaran <[email protected]>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 <[email protected]>
Reviewed-by: Matthew Auld <[email protected]>
--- 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);
