If we can prevent stray writes from landing in the scratch page, we can
reuse the same page and same scratch PT for all contexts without fear of
information leaks and side-channels.
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Matthew Auld <matthew.auld@xxxxxxxxx>
Reviewed-by: Matthew Auld <matthew.auld@xxxxxxxxx>
---
drivers/gpu/drm/i915/i915_gem_gtt.c | 30 +++++++++++++++++++++++++----
1 file changed, 26 insertions(+), 4 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
index afe45cbcd762..9ea024395d49 100644
--- a/drivers/gpu/drm/i915/i915_gem_gtt.c
+++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
@@ -580,10 +580,9 @@ setup_scratch_page(struct i915_address_space *vm, gfp_t gfp)
* region, including any PTEs which happen to point to scratch.
*
* This is only relevant for the 48b PPGTT where we support
- * huge-gtt-pages, see also i915_vma_insert().
- *
- * TODO: we should really consider write-protecting the scratch-page and
- * sharing between ppgtt
+ * huge-gtt-pages, see also i915_vma_insert(). However, as we share the
+ * scratch (read-only) between all vm, we create one 64k scratch page
+ * for all.
*/
size = I915_GTT_PAGE_SIZE_4K;
if (i915_vm_is_48bit(vm) &&
@@ -1209,6 +1208,26 @@ static int gen8_init_scratch(struct i915_address_space *vm)
{
int ret;
+ /*
+ * If everybody agrees to not to write into the scratch page,
+ * we can reuse it for all vm, keeping contexts and processes separate.
+ */
+ if (vm->has_read_only &&
+ vm->i915->kernel_context &&
+ vm->i915->kernel_context->ppgtt) {
+ struct i915_address_space *clone =
+ &vm->i915->kernel_context->ppgtt->vm;
+
+ GEM_BUG_ON(!clone->has_read_only);
+
+ vm->scratch_page.order = clone->scratch_page.order;
+ vm->scratch_pte = clone->scratch_pte;
+ vm->scratch_pt = clone->scratch_pt;
+ vm->scratch_pd = clone->scratch_pd;
+ vm->scratch_pdp = clone->scratch_pdp;
+ return 0;
+ }
+
ret = setup_scratch_page(vm, __GFP_HIGHMEM);
if (ret)
return ret;
@@ -1289,6 +1308,9 @@ static int gen8_ppgtt_notify_vgt(struct i915_hw_ppgtt *ppgtt, bool create)
static void gen8_free_scratch(struct i915_address_space *vm)
{
+ if (!vm->scratch_page.daddr)
+ return;
+
if (use_4lvl(vm))
free_pdp(vm, vm->scratch_pdp);
free_pd(vm, vm->scratch_pd);
--
2.19.1
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