Re: [PATCH 3/4] drm/i915: Flush all user surfaces prior to first use

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Francisco Jerez (2019-07-24 21:37:24)
> Chris Wilson <chris@chris-wilson.co.uk> writes:
> 
> > Since userspace has the ability to bypass the CPU cache from within its
> > unprivileged command stream, we have to flush the CPU cache to memory
> > in order to overwrite the previous contents on creation.
> >
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> > Cc: stablevger.kernel.org
> > ---
> >  drivers/gpu/drm/i915/gem/i915_gem_shmem.c | 26 ++++++-----------------
> >  1 file changed, 7 insertions(+), 19 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> > index d2a1158868e7..f752b326d399 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> > @@ -459,7 +459,6 @@ i915_gem_object_create_shmem(struct drm_i915_private *i915, u64 size)
> >  {
> >       struct drm_i915_gem_object *obj;
> >       struct address_space *mapping;
> > -     unsigned int cache_level;
> >       gfp_t mask;
> >       int ret;
> >  
> > @@ -498,24 +497,13 @@ i915_gem_object_create_shmem(struct drm_i915_private *i915, u64 size)
> >       obj->write_domain = I915_GEM_DOMAIN_CPU;
> >       obj->read_domains = I915_GEM_DOMAIN_CPU;
> >  
> > -     if (HAS_LLC(i915))
> > -             /* On some devices, we can have the GPU use the LLC (the CPU
> > -              * cache) for about a 10% performance improvement
> > -              * compared to uncached.  Graphics requests other than
> > -              * display scanout are coherent with the CPU in
> > -              * accessing this cache.  This means in this mode we
> > -              * don't need to clflush on the CPU side, and on the
> > -              * GPU side we only need to flush internal caches to
> > -              * get data visible to the CPU.
> > -              *
> > -              * However, we maintain the display planes as UC, and so
> > -              * need to rebind when first used as such.
> > -              */
> > -             cache_level = I915_CACHE_LLC;
> > -     else
> > -             cache_level = I915_CACHE_NONE;
> > -
> > -     i915_gem_object_set_cache_coherency(obj, cache_level);
> > +     /*
> > +      * Note that userspace has control over cache-bypass
> > +      * via its command stream, so even on LLC architectures
> > +      * we have to flush out the CPU cache to memory to
> > +      * clear previous contents.
> > +      */
> > +     i915_gem_object_set_cache_coherency(obj, I915_CACHE_NONE);
> >  
> 
> I'm not sure if you've seen my comments about this in an internal thread
> you were CC'ed to: I don't think this patch will have the intended
> effect.  The various clflushes this could trigger before the first use
> of the buffer are conditional on "obj->cache_dirty &
> ~obj->cache_coherent", which will always be false on LLC platforms
> AFAICT, because on those platforms i915_gem_object_set_cache_coherency()
> will always set bit 0 of obj->cache_coherent.

You only need to flush the stale written-to cachelines, so that the page
content is correct on reuse of the foreign struct page. After that point,
the CPU cache is managed by the client.
-Chris
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