[2/3] drm/i915: Introduce guard pages to i915_vma

Commit Message

Andi Shyti Nov. 9, 2022, 4:49 p.m. UTC

From: Chris Wilson <[e-mail]>

Introduce the concept of padding the i915_vma with guard pages before
and after. The major consequence is that all ordinary uses of i915_vma
must use i915_vma_offset/i915_vma_size and not i915_vma.node.start/size
directly, as the drm_mm_node will include the guard pages that surround
our object.

The biggest connundrum is how exactly to mix requesting a fixed address
with guard pages, particularly through the existing uABI. The user does
not know about guard pages, so such must be transparent to the user, and
so the execobj.offset must be that of the object itself excluding the
guard. So a PIN_OFFSET_FIXED must then be exclusive of the guard pages.
The caveat is that some placements will be impossible with guard pages,
as wrap arounds need to be avoided, and the vma itself will require a
larger node. We must not report EINVAL but ENOSPC as these are unavailable
locations within the GTT rather than conflicting user requirements.

In the next patch, we start using guard pages for scanout objects. While
these are limited to GGTT vma, on a few platforms these vma (or at least
an alias of the vma) is shared with userspace, so we may leak the
existence of such guards if we are not careful to ensure that the
execobj.offset is transparent and excludes the guards. (On such platforms
like ivb, without full-ppgtt, userspace has to use relocations so the
presence of more untouchable regions within its GTT such be of no further
issue.)

Signed-off-by: Chris Wilson <[e-mail]>
Cc: Matthew Auld <[e-mail]>
Reviewed-by: Matthew Auld <[e-mail]>
Signed-off-by: Tvrtko Ursulin <[e-mail]>
Signed-off-by: Andi Shyti <[e-mail]>
---
 drivers/gpu/drm/i915/gt/intel_ggtt.c  | 12 ++++++++++--
 drivers/gpu/drm/i915/i915_gem_gtt.h   |  1 +
 drivers/gpu/drm/i915/i915_vma.c       | 28 ++++++++++++++++++++++-----
 drivers/gpu/drm/i915/i915_vma.h       |  5 +++--
 drivers/gpu/drm/i915/i915_vma_types.h |  3 ++-
 5 files changed, 39 insertions(+), 10 deletions(-)

Patch

diff --git a/drivers/gpu/drm/i915/gt/intel_ggtt.c b/drivers/gpu/drm/i915/gt/intel_ggtt.c
index 6549b8e3adbfc..717135f5bf5a6 100644
--- a/drivers/gpu/drm/i915/gt/intel_ggtt.c
+++ b/drivers/gpu/drm/i915/gt/intel_ggtt.c
@@ -266,8 +266,12 @@  static void gen8_ggtt_insert_entries(struct i915_address_space *vm,
 
 	gte = (gen8_pte_t __iomem *)ggtt->gsm;
 	gte += vma->node.start / I915_GTT_PAGE_SIZE;
-	end = gte + vma->node.size / I915_GTT_PAGE_SIZE;
 
+	end = gte + vma->guard / I915_GTT_PAGE_SIZE;
+	while (gte < end)
+		gen8_set_pte(gte++, vm->scratch[0]->encode);
+
+	end += (vma->node.size - vma->guard) / I915_GTT_PAGE_SIZE;
 	for_each_sgt_daddr(addr, iter, vma->pages)
 		gen8_set_pte(gte++, pte_encode | addr);
 	GEM_BUG_ON(gte > end);
@@ -317,8 +321,12 @@  static void gen6_ggtt_insert_entries(struct i915_address_space *vm,
 
 	gte = (gen6_pte_t __iomem *)ggtt->gsm;
 	gte += vma->node.start / I915_GTT_PAGE_SIZE;
-	end = gte + vma->node.size / I915_GTT_PAGE_SIZE;
 
+	end = gte + vma->guard / I915_GTT_PAGE_SIZE;
+	while (gte < end)
+		gen8_set_pte(gte++, vm->scratch[0]->encode);
+
+	end += (vma->node.size - vma->guard) / I915_GTT_PAGE_SIZE;
 	for_each_sgt_daddr(addr, iter, vma->pages)
 		iowrite32(vm->pte_encode(addr, level, flags), gte++);
 	GEM_BUG_ON(gte > end);
diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.h b/drivers/gpu/drm/i915/i915_gem_gtt.h
index c9b0ee5e1d237..f3ae9afdee158 100644
--- a/drivers/gpu/drm/i915/i915_gem_gtt.h
+++ b/drivers/gpu/drm/i915/i915_gem_gtt.h
@@ -41,6 +41,7 @@  int i915_gem_gtt_insert(struct i915_address_space *vm,
 #define PIN_HIGH		BIT_ULL(5)
 #define PIN_OFFSET_BIAS		BIT_ULL(6)
 #define PIN_OFFSET_FIXED	BIT_ULL(7)
+#define PIN_OFFSET_GUARD	BIT_ULL(8)
 
 #define PIN_GLOBAL		BIT_ULL(10) /* I915_VMA_GLOBAL_BIND */
 #define PIN_USER		BIT_ULL(11) /* I915_VMA_LOCAL_BIND */
diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
index 4b4a6dcdc676b..bf0f8d7e13405 100644
--- a/drivers/gpu/drm/i915/i915_vma.c
+++ b/drivers/gpu/drm/i915/i915_vma.c
@@ -633,7 +633,7 @@  bool i915_gem_valid_gtt_space(struct i915_vma *vma, unsigned long color)
 static int
 i915_vma_insert(struct i915_vma *vma, u64 size, u64 alignment, u64 flags)
 {
-	unsigned long color;
+	unsigned long color, guard;
 	u64 start, end;
 	int ret;
 
@@ -641,7 +641,7 @@  i915_vma_insert(struct i915_vma *vma, u64 size, u64 alignment, u64 flags)
 	GEM_BUG_ON(drm_mm_node_allocated(&vma->node));
 
 	size = max(size, vma->size);
-	alignment = max(alignment, vma->display_alignment);
+	alignment = max_t(typeof(alignment), alignment, vma->display_alignment);
 	if (flags & PIN_MAPPABLE) {
 		size = max_t(typeof(size), size, vma->fence_size);
 		alignment = max_t(typeof(alignment),
@@ -652,6 +652,9 @@  i915_vma_insert(struct i915_vma *vma, u64 size, u64 alignment, u64 flags)
 	GEM_BUG_ON(!IS_ALIGNED(alignment, I915_GTT_MIN_ALIGNMENT));
 	GEM_BUG_ON(!is_power_of_2(alignment));
 
+	guard = vma->guard; /* retain guard across rebinds */
+	guard = ALIGN(guard, alignment);
+
 	start = flags & PIN_OFFSET_BIAS ? flags & PIN_OFFSET_MASK : 0;
 	GEM_BUG_ON(!IS_ALIGNED(start, I915_GTT_PAGE_SIZE));
 
@@ -661,12 +664,13 @@  i915_vma_insert(struct i915_vma *vma, u64 size, u64 alignment, u64 flags)
 	if (flags & PIN_ZONE_4G)
 		end = min_t(u64, end, (1ULL << 32) - I915_GTT_PAGE_SIZE);
 	GEM_BUG_ON(!IS_ALIGNED(end, I915_GTT_PAGE_SIZE));
+	GEM_BUG_ON(2 * guard > end);
 
 	/* If binding the object/GGTT view requires more space than the entire
 	 * aperture has, reject it early before evicting everything in a vain
 	 * attempt to find space.
 	 */
-	if (size > end) {
+	if (size > end - 2 * guard) {
 		DRM_DEBUG("Attempting to bind an object larger than the aperture: request=%llu > %s aperture=%llu\n",
 			  size, flags & PIN_MAPPABLE ? "mappable" : "total",
 			  end);
@@ -679,16 +683,29 @@  i915_vma_insert(struct i915_vma *vma, u64 size, u64 alignment, u64 flags)
 
 	if (flags & PIN_OFFSET_FIXED) {
 		u64 offset = flags & PIN_OFFSET_MASK;
+
 		if (!IS_ALIGNED(offset, alignment) ||
 		    range_overflows(offset, size, end))
 			return -EINVAL;
 
+		/*
+		 * The caller knows not of the guard added by others and
+		 * requests for the offset of the start of its buffer
+		 * to be fixed, which may not be the same as the position
+		 * of the vma->node due to the guard pages.
+		 */
+		if (offset < guard || offset + size > end - guard)
+			return -ENOSPC;
+
 		ret = i915_gem_gtt_reserve(vma->vm, &vma->node,
-					   size, offset, color,
-					   flags);
+					   size + 2 * guard,
+					   offset - guard,
+					   color, flags);
 		if (ret)
 			return ret;
 	} else {
+		size += 2 * guard;
+
 		/*
 		 * We only support huge gtt pages through the 48b PPGTT,
 		 * however we also don't want to force any alignment for
@@ -735,6 +752,7 @@  i915_vma_insert(struct i915_vma *vma, u64 size, u64 alignment, u64 flags)
 	GEM_BUG_ON(!i915_gem_valid_gtt_space(vma, color));
 
 	list_add_tail(&vma->vm_link, &vma->vm->bound_list);
+	vma->guard = guard;
 
 	return 0;
 }
diff --git a/drivers/gpu/drm/i915/i915_vma.h b/drivers/gpu/drm/i915/i915_vma.h
index caf6e251782d8..cb5189922ab8a 100644
--- a/drivers/gpu/drm/i915/i915_vma.h
+++ b/drivers/gpu/drm/i915/i915_vma.h
@@ -121,12 +121,13 @@  static inline bool i915_vma_is_closed(const struct i915_vma *vma)
 static inline u64 i915_vma_size(const struct i915_vma *vma)
 {
 	GEM_BUG_ON(!drm_mm_node_allocated(&vma->node));
-	return vma->node.size;
+	return vma->node.size - 2 * vma->guard;
 }
 
 static inline u64 __i915_vma_offset(const struct i915_vma *vma)
 {
-	return vma->node.start;
+	/* The actual start of the vma->pages is after the guard pages. */
+	return vma->node.start + vma->guard;
 }
 
 static inline u64 i915_vma_offset(const struct i915_vma *vma)
diff --git a/drivers/gpu/drm/i915/i915_vma_types.h b/drivers/gpu/drm/i915/i915_vma_types.h
index 4ee6e54799f48..07c72d623c40f 100644
--- a/drivers/gpu/drm/i915/i915_vma_types.h
+++ b/drivers/gpu/drm/i915/i915_vma_types.h
@@ -196,14 +196,15 @@  struct i915_vma {
 	struct i915_fence_reg *fence;
 
 	u64 size;
-	u64 display_alignment;
 	struct i915_page_sizes page_sizes;
 
 	/* mmap-offset associated with fencing for this vma */
 	struct i915_mmap_offset	*mmo;
 
+	u32 guard; /* padding allocated around vma->pages within the node */
 	u32 fence_size;
 	u32 fence_alignment;
+	u32 display_alignment;
 
 	/**
 	 * Count of the number of times this vma has been opened by different