[RFC PATCH 2/4] drm/xe: Strict migration policy for atomic SVM faults

[Matthew Brost]

Mixing GPU and CPU atomics does not work unless a strict migration
policy of GPU atomics must be device memory. Enforce a policy of must be
in VRAM with a retry loop of 2 attempts, if retry loop fails abort
fault.

Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray at intel.com>
Signed-off-by: Matthew Brost <matthew.brost at intel.com>
---
 drivers/gpu/drm/xe/xe_svm.c | 57 ++++++++++++++++++++++++++++---------
 drivers/gpu/drm/xe/xe_svm.h |  5 ----
 2 files changed, 44 insertions(+), 18 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_svm.c b/drivers/gpu/drm/xe/xe_svm.c
index 56b18a293bbc..ec61af659a13 100644
--- a/drivers/gpu/drm/xe/xe_svm.c
+++ b/drivers/gpu/drm/xe/xe_svm.c
@@ -726,6 +726,35 @@ static int xe_svm_alloc_vram(struct xe_vm *vm, struct xe_tile *tile,
 }
 #endif
 
+static bool supports_4K_migration(struct xe_device *xe)
+{
+	if (xe->info.vram_flags & XE_VRAM_FLAGS_NEED64K)
+		return false;
+
+	return true;
+}
+
+static bool xe_svm_range_needs_migrate_to_vram(struct xe_svm_range *range,
+					       struct xe_vma *vma)
+{
+	struct xe_vm *vm = range_to_vm(&range->base);
+	u64 range_size = xe_svm_range_size(range);
+
+	if (!range->base.flags.migrate_devmem)
+		return false;
+
+	if (xe_svm_range_in_vram(range)) {
+		drm_dbg(&vm->xe->drm, "Range is already in VRAM\n");
+		return false;
+	}
+
+	if (range_size <= SZ_64K && !supports_4K_migration(vm->xe)) {
+		drm_dbg(&vm->xe->drm, "Platform doesn't support SZ_4K range migration\n");
+		return false;
+	}
+
+	return true;
+}
 
 /**
  * xe_svm_handle_pagefault() - SVM handle page fault
@@ -750,12 +779,14 @@ int xe_svm_handle_pagefault(struct xe_vm *vm, struct xe_vma *vma,
 			IS_ENABLED(CONFIG_DRM_XE_DEVMEM_MIRROR),
 		.check_pages_threshold = IS_DGFX(vm->xe) &&
 			IS_ENABLED(CONFIG_DRM_XE_DEVMEM_MIRROR) ? SZ_64K : 0,
+		.vram_only = atomic,
 	};
 	struct xe_svm_range *range;
 	struct drm_gpusvm_range *r;
 	struct drm_exec exec;
 	struct dma_fence *fence;
 	struct xe_tile *tile = gt_to_tile(gt);
+	int migrate_try_count = 2;
 	ktime_t end = 0;
 	int err;
 
@@ -782,18 +813,21 @@ int xe_svm_handle_pagefault(struct xe_vm *vm, struct xe_vma *vma,
 
 	range_debug(range, "PAGE FAULT");
 
-	/* XXX: Add migration policy, for now migrate range once */
-	if (!range->skip_migrate && range->base.flags.migrate_devmem &&
-	    xe_svm_range_size(range) >= SZ_64K) {
-		range->skip_migrate = true;
-
+	if (--migrate_try_count >= 0 &&
+	    xe_svm_range_needs_migrate_to_vram(range, vma)) {
 		err = xe_svm_alloc_vram(vm, tile, range, &ctx);
 		if (err) {
-			drm_dbg(&vm->xe->drm,
-				"VRAM allocation failed, falling back to "
-				"retrying fault, asid=%u, errno=%pe\n",
-				vm->usm.asid, ERR_PTR(err));
-			goto retry;
+			if (migrate_try_count || !ctx.vram_only) {
+				drm_dbg(&vm->xe->drm,
+					"VRAM allocation failed, falling back to retrying fault, asid=%u, errno=%pe\n",
+					vm->usm.asid, ERR_PTR(err));
+				goto retry;
+			} else {
+				drm_err(&vm->xe->drm,
+					"VRAM allocation failed, retry count exceeded, asid=%u, errno=%pe\n",
+					vm->usm.asid, ERR_PTR(err));
+				return err;
+			}
 		}
 	}
 
@@ -843,9 +877,6 @@ int xe_svm_handle_pagefault(struct xe_vm *vm, struct xe_vma *vma,
 	}
 	drm_exec_fini(&exec);
 
-	if (xe_modparam.always_migrate_to_vram)
-		range->skip_migrate = false;
-
 	dma_fence_wait(fence, false);
 	dma_fence_put(fence);
 
diff --git a/drivers/gpu/drm/xe/xe_svm.h b/drivers/gpu/drm/xe/xe_svm.h
index 3d441eb1f7ea..0e1f376a7471 100644
--- a/drivers/gpu/drm/xe/xe_svm.h
+++ b/drivers/gpu/drm/xe/xe_svm.h
@@ -39,11 +39,6 @@ struct xe_svm_range {
 	 * range. Protected by GPU SVM notifier lock.
 	 */
 	u8 tile_invalidated;
-	/**
-	 * @skip_migrate: Skip migration to VRAM, protected by GPU fault handler
-	 * locking.
-	 */
-	u8 skip_migrate	:1;
 };
 
 /**
-- 
2.34.1