Just to be clear, is this just adding a CLFLUSH or is it actually changing
the default caching state of buffers from CACHED to NONE?  If it's actually
changing the default state, that's going to break userspace badly.

--Jason

On Fri, Jul 19, 2019 at 5:21 AM Chris Wilson <chris@chris-wilson.co.uk>
wrote:

> Quoting Lionel Landwerlin (2019-07-19 11:18:42)
> > On 18/07/2019 17:54, Chris Wilson wrote:
> > > 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);
> > >
> > >       trace_i915_gem_object_create(obj);
> > >
> >
> > Does i915_drm.h needs updating? :
> >
> >
> > /**
> >   * I915_CACHING_CACHED
> >   *
> >   * GPU access is coherent with cpu caches and furthermore the data is
> > cached in
> >   * last-level caches shared between cpu cores and the gpu GT. Default on
> >   * machines with HAS_LLC.
> >   */
> > #define I915_CACHING_CACHED             1
>
> Sneaky. Thanks,
> -Chris
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