[3/9] drm/i915: extract bypass-llc check into helper

Commit Message

Matthew Auld Oct. 18, 2021, 5:45 p.m. UTC

It looks like we will need this in some more places, so extract as a
helper.

Signed-off-by: Matthew Auld <[e-mail]>
Cc: Thomas Hellström <[e-mail]>
---
 drivers/gpu/drm/i915/gem/i915_gem_object.c | 26 ++++++++++++++++++++++
 drivers/gpu/drm/i915/gem/i915_gem_object.h |  1 +
 drivers/gpu/drm/i915/gem/i915_gem_shmem.c  | 17 +-------------
 3 files changed, 28 insertions(+), 16 deletions(-)

Comments

Thomas Hellstrom Oct. 20, 2021, 2:38 p.m. UTC | #1

On Mon, 2021-10-18 at 18:45 +0100, Matthew Auld wrote:
> It looks like we will need this in some more places, so extract as a
> helper.
> 
> Signed-off-by: Matthew Auld <[e-mail]>
> Cc: Thomas Hellström <[e-mail]>
> ---
>  drivers/gpu/drm/i915/gem/i915_gem_object.c | 26
> ++++++++++++++++++++++
>  drivers/gpu/drm/i915/gem/i915_gem_object.h |  1 +
>  drivers/gpu/drm/i915/gem/i915_gem_shmem.c  | 17 +-------------
>  3 files changed, 28 insertions(+), 16 deletions(-)
> 
Reviewed-by: Thomas Hellström <[e-mail]>

Patch

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.c b/drivers/gpu/drm/i915/gem/i915_gem_object.c
index 76ce6a1500bc..1e426a42a36c 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_object.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_object.c
@@ -128,6 +128,32 @@  void i915_gem_object_set_cache_coherency(struct drm_i915_gem_object *obj,
 		!(obj->cache_coherent & I915_BO_CACHE_COHERENT_FOR_WRITE);
 }
 
+bool i915_gem_object_can_bypass_llc(struct drm_i915_gem_object *obj)
+{
+	struct drm_i915_private *i915 = to_i915(obj->base.dev);
+
+	/*
+	 * This is purely from a security perspective, so we simply don't care
+	 * about non-userspace objects being able to bypass the LLC.
+	 */
+	if (!(obj->flags & I915_BO_ALLOC_USER))
+		return false;
+
+	/*
+	 * EHL and JSL add the 'Bypass LLC' MOCS entry, which should make it
+	 * possible for userspace to bypass the GTT caching bits set by the
+	 * kernel, as per the given object cache_level. This is troublesome
+	 * since the heavy flush we apply when first gathering the pages is
+	 * skipped if the kernel thinks the object is coherent with the GPU. As
+	 * a result it might be possible to bypass the cache and read the
+	 * contents of the page directly, which could be stale data. If it's
+	 * just a case of userspace shooting themselves in the foot then so be
+	 * it, but since i915 takes the stance of always zeroing memory before
+	 * handing it to userspace, we need to prevent this.
+	 */
+	return IS_JSL_EHL(i915);
+}
+
 static void i915_gem_close_object(struct drm_gem_object *gem, struct drm_file *file)
 {
 	struct drm_i915_gem_object *obj = to_intel_bo(gem);
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.h b/drivers/gpu/drm/i915/gem/i915_gem_object.h
index 9df3ee60604e..59201801cec5 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_object.h
+++ b/drivers/gpu/drm/i915/gem/i915_gem_object.h
@@ -514,6 +514,7 @@  i915_gem_object_finish_access(struct drm_i915_gem_object *obj)
 
 void i915_gem_object_set_cache_coherency(struct drm_i915_gem_object *obj,
 					 unsigned int cache_level);
+bool i915_gem_object_can_bypass_llc(struct drm_i915_gem_object *obj);
 void i915_gem_object_flush_if_display(struct drm_i915_gem_object *obj);
 void i915_gem_object_flush_if_display_locked(struct drm_i915_gem_object *obj);
 
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
index 11f072193f3b..cf11aa7e08a0 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
@@ -182,22 +182,7 @@  static int shmem_get_pages(struct drm_i915_gem_object *obj)
 	if (i915_gem_object_needs_bit17_swizzle(obj))
 		i915_gem_object_do_bit_17_swizzle(obj, st);
 
-	/*
-	 * EHL and JSL add the 'Bypass LLC' MOCS entry, which should make it
-	 * possible for userspace to bypass the GTT caching bits set by the
-	 * kernel, as per the given object cache_level. This is troublesome
-	 * since the heavy flush we apply when first gathering the pages is
-	 * skipped if the kernel thinks the object is coherent with the GPU. As
-	 * a result it might be possible to bypass the cache and read the
-	 * contents of the page directly, which could be stale data. If it's
-	 * just a case of userspace shooting themselves in the foot then so be
-	 * it, but since i915 takes the stance of always zeroing memory before
-	 * handing it to userspace, we need to prevent this.
-	 *
-	 * By setting cache_dirty here we make the clflush in set_pages
-	 * unconditional on such platforms.
-	 */
-	if (IS_JSL_EHL(i915) && obj->flags & I915_BO_ALLOC_USER)
+	if (i915_gem_object_can_bypass_llc(obj))
 		obj->cache_dirty = true;
 
 	__i915_gem_object_set_pages(obj, st, sg_page_sizes);