From 8a9a982767b7c89b2f23290e4f0f21f194b79dfe Mon Sep 17 00:00:00 2001 From: Daniele Ceraolo Spurio Date: Tue, 27 Aug 2019 11:58:05 -0700 Subject: [PATCH] drm/i915: use a separate context for gpu relocs The CS pre-parser can pre-fetch commands across memory sync points and starting from gen12 it is able to pre-fetch across BB_START and BB_END boundaries as well, so when we emit gpu relocs the pre-parser might fetch the target location of the reloc before the memory write lands. The parser can't pre-fetch across the ctx switch, so we use a separate context to guarantee that the memory is synchronized before the parser can get to it. Note that there is no risk of the CS doing a lite restore from the reloc context to the user context, even if the two have the same hw_id, because since gen11 the CS also checks the LRCA when deciding if it can lite-restore. v2: limit new context to gen12+, release in eb_destroy, add a comment in emit_fini_breadcrumb (Chris). Suggested-by: Chris Wilson Signed-off-by: Daniele Ceraolo Spurio Cc: Chris Wilson Reviewed-by: Chris Wilson Signed-off-by: Chris Wilson Link: https://patchwork.freedesktop.org/patch/msgid/20190827185805.21799-1-daniele.ceraolospurio@intel.com --- .../gpu/drm/i915/gem/i915_gem_execbuffer.c | 30 ++++++++++++++++++- drivers/gpu/drm/i915/gt/intel_lrc.c | 18 +++++++++++ 2 files changed, 47 insertions(+), 1 deletion(-) diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c index 5a2238d44423..8fbb454cfd6b 100644 --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c @@ -252,6 +252,7 @@ struct i915_execbuffer { bool has_fence : 1; bool needs_unfenced : 1; + struct intel_context *ce; struct i915_request *rq; u32 *rq_cmd; unsigned int rq_size; @@ -880,6 +881,9 @@ static void eb_destroy(const struct i915_execbuffer *eb) { GEM_BUG_ON(eb->reloc_cache.rq); + if (eb->reloc_cache.ce) + intel_context_put(eb->reloc_cache.ce); + if (eb->lut_size > 0) kfree(eb->buckets); } @@ -903,6 +907,7 @@ static void reloc_cache_init(struct reloc_cache *cache, cache->has_fence = cache->gen < 4; cache->needs_unfenced = INTEL_INFO(i915)->unfenced_needs_alignment; cache->node.allocated = false; + cache->ce = NULL; cache->rq = NULL; cache->rq_size = 0; } @@ -1168,7 +1173,7 @@ static int __reloc_gpu_alloc(struct i915_execbuffer *eb, if (err) goto err_unmap; - rq = i915_request_create(eb->context); + rq = intel_context_create_request(cache->ce); if (IS_ERR(rq)) { err = PTR_ERR(rq); goto err_unpin; @@ -1239,6 +1244,29 @@ static u32 *reloc_gpu(struct i915_execbuffer *eb, if (!intel_engine_can_store_dword(eb->engine)) return ERR_PTR(-ENODEV); + if (!cache->ce) { + struct intel_context *ce; + + /* + * The CS pre-parser can pre-fetch commands across + * memory sync points and starting gen12 it is able to + * pre-fetch across BB_START and BB_END boundaries + * (within the same context). We therefore use a + * separate context gen12+ to guarantee that the reloc + * writes land before the parser gets to the target + * memory location. + */ + if (cache->gen >= 12) + ce = intel_context_create(eb->context->gem_context, + eb->engine); + else + ce = intel_context_get(eb->context); + if (IS_ERR(ce)) + return ERR_CAST(ce); + + cache->ce = ce; + } + err = __reloc_gpu_alloc(eb, vma, len); if (unlikely(err)) return ERR_PTR(err); diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c index cfbdcca76ff0..a141e9e37bf7 100644 --- a/drivers/gpu/drm/i915/gt/intel_lrc.c +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c @@ -2931,6 +2931,24 @@ static u32 *gen8_emit_fini_breadcrumb_rcs(struct i915_request *request, u32 *cs) return gen8_emit_fini_breadcrumb_footer(request, cs); } +/* + * Note that the CS instruction pre-parser will not stall on the breadcrumb + * flush and will continue pre-fetching the instructions after it before the + * memory sync is completed. On pre-gen12 HW, the pre-parser will stop at + * BB_START/END instructions, so, even though we might pre-fetch the pre-amble + * of the next request before the memory has been flushed, we're guaranteed that + * we won't access the batch itself too early. + * However, on gen12+ the parser can pre-fetch across the BB_START/END commands, + * so, if the current request is modifying an instruction in the next request on + * the same intel_context, we might pre-fetch and then execute the pre-update + * instruction. To avoid this, the users of self-modifying code should either + * disable the parser around the code emitting the memory writes, via a new flag + * added to MI_ARB_CHECK, or emit the writes from a different intel_context. For + * the in-kernel use-cases we've opted to use a separate context, see + * reloc_gpu() as an example. + * All the above applies only to the instructions themselves. Non-inline data + * used by the instructions is not pre-fetched. + */ static u32 *gen11_emit_fini_breadcrumb_rcs(struct i915_request *request, u32 *cs) { -- 2.30.2