Re: [PATCH v2] drm/i915/dmabuf: avoid global wbinvd on dma-buf import

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Thu, 2026-08-20 at 10:23 +0100, Matthew Auld wrote:
> On 16/08/2026 02:45, Prabhakaran, Krishna wrote:
> > 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))) {
> 
> I think we can bump this now for dg2? I think we treat dgfx as always
> coherent with system memory. So maybe s/IS_DG1/IS_DGFX/ in a separate
> patch? Pretty sure the rest of the driver is the same.

+1

> 
> > +		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();
> 
> Do we need the flush under the else here? If it's not placed in
> system 
> memory what is this flushing, from i915 pov?
> 
> > +		} 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();

The need for this flush is a bit unfortunate. In theory iomem can be
mapped and wiped write-back, But I'm not sure how common that is...



> > +				dma_buf_vunmap(dma_buf, &map);



> > +			} else {
> > +				wbinvd_on_all_cpus();

Random idea, would it make sense to insert a GPU CLFLUSH into the
pipeline using MI_CLFLUSH (on hardware that supports it) or an
uncached but coherent MOCS / GPU PAT setting dummy blit?

Thanks,
Thomas


> > +			}
> > +		}
> > +	}
> >   
> >   	__i915_gem_object_set_pages(obj, sgt);
> >   
> > 
> > base-commit: 682ea2d28d18bb06f9fc663cb5ab7e80dc0e606a