Add some details around non-LLC platforms and cflushing, when dealing with the flush-on-acquire, which is potentially security sensitive.
Signed-off-by: Matthew Auld matthew.auld@intel.com Cc: Thomas Hellström thomas.hellstrom@linux.intel.com Cc: Daniel Vetter daniel@ffwll.ch --- .../gpu/drm/i915/gem/i915_gem_execbuffer.c | 11 ++++++++ .../gpu/drm/i915/gem/i915_gem_object_types.h | 27 +++++++++++++++++++ 2 files changed, 38 insertions(+)
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c index 1231224728e4..9c323666bd7c 100644 --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c @@ -1933,6 +1933,17 @@ static int eb_move_to_gpu(struct i915_execbuffer *eb) * !(obj->cache_coherent & I915_BO_CACHE_COHERENT_FOR_READ) * but gcc's optimiser doesn't handle that as well and emits * two jumps instead of one. Maybe one day... + * + * FIXME: There is also sync flushing in set_pages(), which + * serves a different purpose(some of the time at least). + * + * We should consider: + * + * 1. Rip out the async flush code. + * + * 2. Or make the sync flushing use the async clflush path + * using mandatory fences underneath. Currently the below + * async flush happens after we bind the object. */ if (unlikely(obj->cache_dirty & ~obj->cache_coherent)) { if (i915_gem_clflush_object(obj, 0)) diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object_types.h b/drivers/gpu/drm/i915/gem/i915_gem_object_types.h index 7c3da4e3e737..da85169006d4 100644 --- a/drivers/gpu/drm/i915/gem/i915_gem_object_types.h +++ b/drivers/gpu/drm/i915/gem/i915_gem_object_types.h @@ -427,6 +427,33 @@ struct drm_i915_gem_object { * can freely bypass the CPU cache when touching the pages with the GPU, * where the kernel is completely unaware. On such platform we need * apply the sledgehammer-on-acquire regardless of the @cache_coherent. + * + * Special care is taken on non-LLC platforms, to prevent potential + * information leak. The driver currently ensures: + * + * 1. All userspace objects, by default, have @cache_level set as + * I915_CACHE_NONE. The only exception is userptr objects, where we + * instead force I915_CACHE_LLC, but we also don't allow userspace to + * ever change the @cache_level for such objects. Another special case + * is dma-buf, which doesn't rely on @cache_dirty, but there we + * always do a forced flush when acquiring the pages, if there is a + * chance that the pages can be read directly from main memory with + * the GPU. + * + * 2. All I915_CACHE_NONE objects have @cache_dirty initially true. + * + * 3. All swapped-out objects(i.e shmem) have @cache_dirty set to + * true. + * + * 4. The @cache_dirty is never freely reset before the initial + * flush, even if userspace adjusts the @cache_level through the + * i915_gem_set_caching_ioctl. + * + * 5. All @cache_dirty objects(including swapped-in) are initially + * flushed with a synchronous call to drm_clflush_sg in + * __i915_gem_object_set_pages. The @cache_dirty can be freely reset + * at this point. All further asynchronous clfushes are never security + * critical, i.e userspace is free to race against itself. */ unsigned int cache_dirty:1;