[igt-dev] [PATCH i-g-t] lib/intel_memory_region: Use separate context for probing offset and alignment

[Zbigniew Kempczyński]

Probing alignment/offset (A/O) in default context works properly only
when there're no processes which competes on same vm space. To avoid
risk that single probe will be called on already used offset in another
process lets use dedicated context for this purpose if possible.

In other words when forking occur without A/O cache filled (subject of
COW) children will exercise A/O individually. Using same default context
leads to risk of probing offset which is in flight in another child
thus we can get different A/O. Such behavior is not allowed as allocator
infrastructure requires same type, strategy and alignment on single vm.
We expect coherent A/O in different children so we try to use separate
context to fill this requirement.

v2: on old gens where're no logical contexts use default context
v3: adding missing condition for context destroy (Matt)

Signed-off-by: Zbigniew Kempczyński <zbigniew.kempczynski at intel.com>
Cc: Matthew Auld <matthew.auld at intel.com>
Reviewed-by: Matthew Auld <matthew.auld at intel.com>
Fixes: https://gitlab.freedesktop.org/drm/intel/-/issues/5729
---
 lib/i915/intel_memory_region.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/lib/i915/intel_memory_region.c b/lib/i915/intel_memory_region.c
index 593f4bedc8..6bf6aab1e6 100644
--- a/lib/i915/intel_memory_region.c
+++ b/lib/i915/intel_memory_region.c
@@ -630,7 +630,7 @@ uint64_t gem_detect_min_start_offset_for_region(int i915, uint32_t region)
 	struct drm_i915_gem_execbuffer2 eb;
 	uint64_t start_offset = 0;
 	uint64_t bb_size = PAGE_SIZE;
-	uint32_t *batch;
+	uint32_t *batch, ctx = 0;
 	uint16_t devid = intel_get_drm_devid(i915);
 	struct cache_entry *entry, *newentry;
 
@@ -640,12 +640,16 @@ uint64_t gem_detect_min_start_offset_for_region(int i915, uint32_t region)
 		goto out;
 	pthread_mutex_unlock(&cache_mutex);
 
+	/* Use separate context if possible to avoid offset overlapping */
+	__gem_context_create(i915, &ctx);
+
 	memset(&obj, 0, sizeof(obj));
 	memset(&eb, 0, sizeof(eb));
 
 	eb.buffers_ptr = to_user_pointer(&obj);
 	eb.buffer_count = 1;
 	eb.flags = I915_EXEC_DEFAULT;
+	eb.rsvd1 = ctx;
 	igt_assert(__gem_create_in_memory_regions(i915, &obj.handle, &bb_size, region) == 0);
 	obj.flags = EXEC_OBJECT_PINNED;
 
@@ -670,6 +674,8 @@ uint64_t gem_detect_min_start_offset_for_region(int i915, uint32_t region)
 		igt_assert(start_offset <= 1ull << 48);
 	}
 	gem_close(i915, obj.handle);
+	if (ctx)
+		gem_context_destroy(i915, ctx);
 
 	newentry = malloc(sizeof(*newentry));
 	if (!newentry)
@@ -770,7 +776,7 @@ uint64_t gem_detect_min_alignment_for_regions(int i915,
 	struct drm_i915_gem_execbuffer2 eb;
 	uint64_t min_alignment = PAGE_SIZE;
 	uint64_t bb_size = PAGE_SIZE, obj_size = PAGE_SIZE;
-	uint32_t *batch;
+	uint32_t *batch, ctx = 0;
 	uint16_t devid = intel_get_drm_devid(i915);
 	struct cache_entry *entry, *newentry;
 
@@ -780,6 +786,9 @@ uint64_t gem_detect_min_alignment_for_regions(int i915,
 		goto out;
 	pthread_mutex_unlock(&cache_mutex);
 
+	/* Use separate context if possible to avoid offset overlapping */
+	__gem_context_create(i915, &ctx);
+
 	memset(obj, 0, sizeof(obj));
 	memset(&eb, 0, sizeof(eb));
 
@@ -787,6 +796,7 @@ uint64_t gem_detect_min_alignment_for_regions(int i915,
 	eb.buffers_ptr = to_user_pointer(obj);
 	eb.buffer_count = ARRAY_SIZE(obj);
 	eb.flags = I915_EXEC_BATCH_FIRST | I915_EXEC_DEFAULT;
+	eb.rsvd1 = ctx;
 	igt_assert(__gem_create_in_memory_regions(i915, &obj[0].handle,
 						  &bb_size, region1) == 0);
 
@@ -815,6 +825,8 @@ uint64_t gem_detect_min_alignment_for_regions(int i915,
 
 	gem_close(i915, obj[0].handle);
 	gem_close(i915, obj[1].handle);
+	if (ctx)
+		gem_context_destroy(i915, ctx);
 
 	newentry = malloc(sizeof(*newentry));
 	if (!newentry)
-- 
2.32.0