drm/i915: Add mechanism to submit a context WA on ring submission

This patch adds framework to submit an arbitrary batchbuffer on each
context switch to clear residual state for render engine on Gen7/7.5
devices.

The idea of always emitting the context and vm setup around each request
is primary to make reset recovery easy, and not require rewriting the
ringbuffer. As each request would set up its own context, leaving it to
the HW to notice and elide no-op context switches, we could restart the
ring at any point, and reorder the requests freely.

However, to avoid emitting clear_residuals() between consecutive requests
in the ringbuffer of the same context, we do want to track the current
context in the ring. In doing so, we need to be careful to only record a
context switch when we are sure the next request will be emitted.

This security mitigation change does not trigger any performance
regression. Performance is on par with current mainline/drm-tip.

v2: Update vm_alias params to point to correct address space "vm" due to
changes made in the patch "f21613797bae98773"

v3-v4: none

Signed-off-by: Mika Kuoppala <[e-mail]>
Signed-off-by: Prathap Kumar Valsan <[e-mail]>
Signed-off-by: Akeem G Abodunrin <[e-mail]>
Cc: Chris Wilson <[e-mail]>
Cc: Balestrieri Francesco <[e-mail]>
Cc: Bloomfield Jon <[e-mail]>
Cc: Dutt Sudeep <[e-mail]>
Reviewed-by: Chris Wilson <[e-mail]>
Signed-off-by: Chris Wilson <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/i915/gt/intel_ring_submission.c

···
 	return rq->engine->emit_flush(rq, EMIT_FLUSH);
 }
 
-static inline int mi_set_context(struct i915_request *rq, u32 flags)
+static inline int mi_set_context(struct i915_request *rq,
+				 struct intel_context *ce,
+				 u32 flags)
 {
 	struct drm_i915_private *i915 = rq->i915;
 	struct intel_engine_cs *engine = rq->engine;
···
 
 	*cs++ = MI_NOOP;
 	*cs++ = MI_SET_CONTEXT;
-	*cs++ = i915_ggtt_offset(rq->context->state) | flags;
+	*cs++ = i915_ggtt_offset(ce->state) | flags;
 	/*
 	 * w/a: MI_SET_CONTEXT must always be followed by MI_NOOP
 	 * WaMiSetContext_Hang:snb,ivb,vlv
···
 	return rq->engine->emit_flush(rq, EMIT_INVALIDATE);
 }
 
+static int clear_residuals(struct i915_request *rq)
+{
+	struct intel_engine_cs *engine = rq->engine;
+	int ret;
+
+	ret = switch_mm(rq, vm_alias(engine->kernel_context->vm));
+	if (ret)
+		return ret;
+
+	if (engine->kernel_context->state) {
+		ret = mi_set_context(rq,
+				     engine->kernel_context,
+				     MI_MM_SPACE_GTT | MI_RESTORE_INHIBIT);
+		if (ret)
+			return ret;
+	}
+
+	ret = engine->emit_bb_start(rq,
+				    engine->wa_ctx.vma->node.start, 0,
+				    0);
+	if (ret)
+		return ret;
+
+	ret = engine->emit_flush(rq, EMIT_FLUSH);
+	if (ret)
+		return ret;
+
+	/* Always invalidate before the next switch_mm() */
+	return engine->emit_flush(rq, EMIT_INVALIDATE);
+}
+
 static int switch_context(struct i915_request *rq)
 {
+	struct intel_engine_cs *engine = rq->engine;
 	struct intel_context *ce = rq->context;
+	void **residuals = NULL;
 	int ret;
 
 	GEM_BUG_ON(HAS_EXECLISTS(rq->i915));
+
+	if (engine->wa_ctx.vma && ce != engine->kernel_context) {
+		if (engine->wa_ctx.vma->private != ce) {
+			ret = clear_residuals(rq);
+			if (ret)
+				return ret;
+
+			residuals = &engine->wa_ctx.vma->private;
+		}
+	}
 
 	ret = switch_mm(rq, vm_alias(ce->vm));
 	if (ret)
···
 	if (ce->state) {
 		u32 flags;
 
-		GEM_BUG_ON(rq->engine->id != RCS0);
+		GEM_BUG_ON(engine->id != RCS0);
 
 		/* For resource streamer on HSW+ and power context elsewhere */
 		BUILD_BUG_ON(HSW_MI_RS_SAVE_STATE_EN != MI_SAVE_EXT_STATE_EN);
···
 		else
 			flags |= MI_RESTORE_INHIBIT;
 
-		ret = mi_set_context(rq, flags);
+		ret = mi_set_context(rq, ce, flags);
 		if (ret)
 			return ret;
 	}
···
 	ret = remap_l3(rq);
 	if (ret)
 		return ret;
+
+	/*
+	 * Now past the point of no return, this request _will_ be emitted.
+	 *
+	 * Or at least this preamble will be emitted, the request may be
+	 * interrupted prior to submitting the user payload. If so, we
+	 * still submit the "empty" request in order to preserve global
+	 * state tracking such as this, our tracking of the current
+	 * dirty context.
+	 */
+	if (residuals) {
+		intel_context_put(*residuals);
+		*residuals = intel_context_get(ce);
+	}
 
 	return 0;
 }
···
 
 	intel_engine_cleanup_common(engine);
 
+	if (engine->wa_ctx.vma) {
+		intel_context_put(engine->wa_ctx.vma->private);
+		i915_vma_unpin_and_release(&engine->wa_ctx.vma, 0);
+	}
+
 	intel_ring_unpin(engine->legacy.ring);
 	intel_ring_put(engine->legacy.ring);
 
···
 	engine->emit_fini_breadcrumb = gen7_xcs_emit_breadcrumb;
 }
 
+static int gen7_ctx_switch_bb_setup(struct intel_engine_cs * const engine,
+				    struct i915_vma * const vma)
+{
+	return 0;
+}
+
+static int gen7_ctx_switch_bb_init(struct intel_engine_cs *engine)
+{
+	struct drm_i915_gem_object *obj;
+	struct i915_vma *vma;
+	int size;
+	int err;
+
+	size = gen7_ctx_switch_bb_setup(engine, NULL /* probe size */);
+	if (size <= 0)
+		return size;
+
+	size = ALIGN(size, PAGE_SIZE);
+	obj = i915_gem_object_create_internal(engine->i915, size);
+	if (IS_ERR(obj))
+		return PTR_ERR(obj);
+
+	vma = i915_vma_instance(obj, engine->gt->vm, NULL);
+	if (IS_ERR(vma)) {
+		err = PTR_ERR(vma);
+		goto err_obj;
+	}
+
+	vma->private = intel_context_create(engine); /* dummy residuals */
+	if (IS_ERR(vma->private)) {
+		err = PTR_ERR(vma->private);
+		goto err_obj;
+	}
+
+	err = i915_vma_pin(vma, 0, 0, PIN_USER | PIN_HIGH);
+	if (err)
+		goto err_private;
+
+	err = gen7_ctx_switch_bb_setup(engine, vma);
+	if (err)
+		goto err_unpin;
+
+	engine->wa_ctx.vma = vma;
+	return 0;
+
+err_unpin:
+	i915_vma_unpin(vma);
+err_private:
+	intel_context_put(vma->private);
+err_obj:
+	i915_gem_object_put(obj);
+	return err;
+}
+
 int intel_ring_submission_setup(struct intel_engine_cs *engine)
 {
 	struct intel_timeline *timeline;
···
 
 	GEM_BUG_ON(timeline->hwsp_ggtt != engine->status_page.vma);
 
+	if (IS_GEN(engine->i915, 7) && engine->class == RENDER_CLASS) {
+		err = gen7_ctx_switch_bb_init(engine);
+		if (err)
+			goto err_ring_unpin;
+	}
+
 	/* Finally, take ownership and responsibility for cleanup! */
 	engine->release = ring_release;
 
 	return 0;
 
+err_ring_unpin:
+	intel_ring_unpin(ring);
 err_ring:
 	intel_ring_put(ring);
 err_timeline_unpin:
···
 	intel_engine_cleanup_common(engine);
 	return err;
 }
+
+#if IS_ENABLED(CONFIG_DRM_I915_SELFTEST)
+#include "selftest_ring_submission.c"
+#endif

drivers/gpu/drm/i915/gt/selftest_ring_submission.c

···
+// SPDX-License-Identifier: MIT
+/*
+ * Copyright © 2020 Intel Corporation
+ */
+
+#include "intel_engine_pm.h"
+#include "selftests/igt_flush_test.h"
+
+static struct i915_vma *create_wally(struct intel_engine_cs *engine)
+{
+	struct drm_i915_gem_object *obj;
+	struct i915_vma *vma;
+	u32 *cs;
+	int err;
+
+	obj = i915_gem_object_create_internal(engine->i915, 4096);
+	if (IS_ERR(obj))
+		return ERR_CAST(obj);
+
+	vma = i915_vma_instance(obj, engine->gt->vm, NULL);
+	if (IS_ERR(vma)) {
+		i915_gem_object_put(obj);
+		return vma;
+	}
+
+	err = i915_vma_pin(vma, 0, 0, PIN_USER | PIN_HIGH);
+	if (err) {
+		i915_gem_object_put(obj);
+		return ERR_PTR(err);
+	}
+
+	cs = i915_gem_object_pin_map(obj, I915_MAP_WC);
+	if (IS_ERR(cs)) {
+		i915_gem_object_put(obj);
+		return ERR_CAST(cs);
+	}
+
+	if (INTEL_GEN(engine->i915) >= 6) {
+		*cs++ = MI_STORE_DWORD_IMM_GEN4;
+		*cs++ = 0;
+	} else if (INTEL_GEN(engine->i915) >= 4) {
+		*cs++ = MI_STORE_DWORD_IMM_GEN4 | MI_USE_GGTT;
+		*cs++ = 0;
+	} else {
+		*cs++ = MI_STORE_DWORD_IMM | MI_MEM_VIRTUAL;
+	}
+	*cs++ = vma->node.start + 4000;
+	*cs++ = STACK_MAGIC;
+
+	*cs++ = MI_BATCH_BUFFER_END;
+	i915_gem_object_unpin_map(obj);
+
+	vma->private = intel_context_create(engine); /* dummy residuals */
+	if (IS_ERR(vma->private)) {
+		vma = ERR_CAST(vma->private);
+		i915_gem_object_put(obj);
+	}
+
+	return vma;
+}
+
+static int context_sync(struct intel_context *ce)
+{
+	struct i915_request *rq;
+	int err = 0;
+
+	rq = intel_context_create_request(ce);
+	if (IS_ERR(rq))
+		return PTR_ERR(rq);
+
+	i915_request_get(rq);
+	i915_request_add(rq);
+
+	if (i915_request_wait(rq, 0, HZ / 5) < 0)
+		err = -ETIME;
+	i915_request_put(rq);
+
+	return err;
+}
+
+static int new_context_sync(struct intel_engine_cs *engine)
+{
+	struct intel_context *ce;
+	int err;
+
+	ce = intel_context_create(engine);
+	if (IS_ERR(ce))
+		return PTR_ERR(ce);
+
+	err = context_sync(ce);
+	intel_context_put(ce);
+
+	return err;
+}
+
+static int mixed_contexts_sync(struct intel_engine_cs *engine, u32 *result)
+{
+	int pass;
+	int err;
+
+	for (pass = 0; pass < 2; pass++) {
+		WRITE_ONCE(*result, 0);
+		err = context_sync(engine->kernel_context);
+		if (err || READ_ONCE(*result)) {
+			if (!err) {
+				pr_err("pass[%d] wa_bb emitted for the kernel context\n",
+				       pass);
+				err = -EINVAL;
+			}
+			return err;
+		}
+
+		WRITE_ONCE(*result, 0);
+		err = new_context_sync(engine);
+		if (READ_ONCE(*result) != STACK_MAGIC) {
+			if (!err) {
+				pr_err("pass[%d] wa_bb *NOT* emitted after the kernel context\n",
+				       pass);
+				err = -EINVAL;
+			}
+			return err;
+		}
+
+		WRITE_ONCE(*result, 0);
+		err = new_context_sync(engine);
+		if (READ_ONCE(*result) != STACK_MAGIC) {
+			if (!err) {
+				pr_err("pass[%d] wa_bb *NOT* emitted for the user context switch\n",
+				       pass);
+				err = -EINVAL;
+			}
+			return err;
+		}
+	}
+
+	return 0;
+}
+
+static int double_context_sync_00(struct intel_engine_cs *engine, u32 *result)
+{
+	struct intel_context *ce;
+	int err, i;
+
+	ce = intel_context_create(engine);
+	if (IS_ERR(ce))
+		return PTR_ERR(ce);
+
+	for (i = 0; i < 2; i++) {
+		WRITE_ONCE(*result, 0);
+		err = context_sync(ce);
+		if (err)
+			break;
+	}
+	intel_context_put(ce);
+	if (err)
+		return err;
+
+	if (READ_ONCE(*result)) {
+		pr_err("wa_bb emitted between the same user context\n");
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
+static int kernel_context_sync_00(struct intel_engine_cs *engine, u32 *result)
+{
+	struct intel_context *ce;
+	int err, i;
+
+	ce = intel_context_create(engine);
+	if (IS_ERR(ce))
+		return PTR_ERR(ce);
+
+	for (i = 0; i < 2; i++) {
+		WRITE_ONCE(*result, 0);
+		err = context_sync(ce);
+		if (err)
+			break;
+
+		err = context_sync(engine->kernel_context);
+		if (err)
+			break;
+	}
+	intel_context_put(ce);
+	if (err)
+		return err;
+
+	if (READ_ONCE(*result)) {
+		pr_err("wa_bb emitted between the same user context [with intervening kernel]\n");
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
+static int __live_ctx_switch_wa(struct intel_engine_cs *engine)
+{
+	struct i915_vma *bb;
+	u32 *result;
+	int err;
+
+	bb = create_wally(engine);
+	if (IS_ERR(bb))
+		return PTR_ERR(bb);
+
+	result = i915_gem_object_pin_map(bb->obj, I915_MAP_WC);
+	if (IS_ERR(result)) {
+		intel_context_put(bb->private);
+		i915_vma_unpin_and_release(&bb, 0);
+		return PTR_ERR(result);
+	}
+	result += 1000;
+
+	engine->wa_ctx.vma = bb;
+
+	err = mixed_contexts_sync(engine, result);
+	if (err)
+		goto out;
+
+	err = double_context_sync_00(engine, result);
+	if (err)
+		goto out;
+
+	err = kernel_context_sync_00(engine, result);
+	if (err)
+		goto out;
+
+out:
+	intel_context_put(engine->wa_ctx.vma->private);
+	i915_vma_unpin_and_release(&engine->wa_ctx.vma, I915_VMA_RELEASE_MAP);
+	return err;
+}
+
+static int live_ctx_switch_wa(void *arg)
+{
+	struct intel_gt *gt = arg;
+	struct intel_engine_cs *engine;
+	enum intel_engine_id id;
+
+	/*
+	 * Exercise the inter-context wa batch.
+	 *
+	 * Between each user context we run a wa batch, and since it may
+	 * have implications for user visible state, we have to check that
+	 * we do actually execute it.
+	 *
+	 * The trick we use is to replace the normal wa batch with a custom
+	 * one that writes to a marker within it, and we can then look for
+	 * that marker to confirm if the batch was run when we expect it,
+	 * and equally important it was wasn't run when we don't!
+	 */
+
+	for_each_engine(engine, gt, id) {
+		struct i915_vma *saved_wa;
+		int err;
+
+		if (!intel_engine_can_store_dword(engine))
+			continue;
+
+		if (IS_GEN_RANGE(gt->i915, 4, 5))
+			continue; /* MI_STORE_DWORD is privileged! */
+
+		saved_wa = fetch_and_zero(&engine->wa_ctx.vma);
+
+		intel_engine_pm_get(engine);
+		err = __live_ctx_switch_wa(engine);
+		intel_engine_pm_put(engine);
+		if (igt_flush_test(gt->i915))
+			err = -EIO;
+
+		engine->wa_ctx.vma = saved_wa;
+		if (err)
+			return err;
+	}
+
+	return 0;
+}
+
+int intel_ring_submission_live_selftests(struct drm_i915_private *i915)
+{
+	static const struct i915_subtest tests[] = {
+		SUBTEST(live_ctx_switch_wa),
+	};
+
+	if (HAS_EXECLISTS(i915))
+		return 0;
+
+	return intel_gt_live_subtests(tests, &i915->gt);
+}

drivers/gpu/drm/i915/selftests/i915_live_selftests.h

···
 selftest(memory_region, intel_memory_region_live_selftests)
 selftest(hangcheck, intel_hangcheck_live_selftests)
 selftest(execlists, intel_execlists_live_selftests)
+selftest(ring_submission, intel_ring_submission_live_selftests)
 selftest(perf, i915_perf_live_selftests)
 /* Here be dragons: keep last to run last! */
 selftest(late_gt_pm, intel_gt_pm_late_selftests)