Re: [PATCH 5/5] drm/msm: Skip tlbinv on unmap from non-current pgtables

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On 8/21/2022 11:49 PM, Rob Clark wrote:
From: Rob Clark <robdclark@xxxxxxxxxxxx>

We can rely on the tlbinv done by CP_SMMU_TABLE_UPDATE in this case.

Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
---
  drivers/gpu/drm/msm/adreno/a6xx_gpu.c |  6 ++++++
  drivers/gpu/drm/msm/msm_iommu.c       | 29 +++++++++++++++++++++++++++
  2 files changed, 35 insertions(+)

diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
index c8ad8aeca777..1ba0ed629549 100644
--- a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
+++ b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
@@ -1180,6 +1180,12 @@ static int hw_init(struct msm_gpu *gpu)
  	/* Always come up on rb 0 */
  	a6xx_gpu->cur_ring = gpu->rb[0];
+ /*
+	 * Note, we cannot assume anything about the state of the SMMU when
+	 * coming back from power collapse, so force a CP_SMMU_TABLE_UPDATE
+	 * on the first submit.  Also, msm_iommu_pagetable_unmap() relies on
+	 * this behavior.
+	 */
  	gpu->cur_ctx_seqno = 0;
/* Enable the SQE_to start the CP engine */
diff --git a/drivers/gpu/drm/msm/msm_iommu.c b/drivers/gpu/drm/msm/msm_iommu.c
index 94c8c09980d1..218074a58081 100644
--- a/drivers/gpu/drm/msm/msm_iommu.c
+++ b/drivers/gpu/drm/msm/msm_iommu.c
@@ -45,8 +45,37 @@ static int msm_iommu_pagetable_unmap(struct msm_mmu *mmu, u64 iova,
  		size -= 4096;
  	}
+ /*
+	 * A CP_SMMU_TABLE_UPDATE is always sent for the first
+	 * submit after resume, and that does a TLB invalidate.
+	 * So we can skip that if the device is not currently
+	 * powered.
+	 */
+	if (!pm_runtime_get_if_in_use(pagetable->parent->dev))
+		goto out;
+
+	/*
+	 * If we are not the current pgtables, we can rely on the
+	 * TLB invalidate done by CP_SMMU_TABLE_UPDATE.
+	 *
+	 * We'll always be racing with the GPU updating ttbr0,
+	 * but there are only two cases:
+	 *
+	 *  + either we are not the the current pgtables and there
+	 *    will be a tlbinv done by the GPU before we are again
+	 *
+	 *  + or we are.. there might have already been a tblinv
+	 *    if we raced with the GPU, but we have to assume the
+	 *    worse and do the tlbinv
+	 */
+	if (adreno_smmu->get_ttbr0(adreno_smmu->cookie) != pagetable->ttbr)
+		goto out_put;
+
  	adreno_smmu->tlb_inv_by_id(adreno_smmu->cookie, pagetable->asid);
+out_put:
+	pm_runtime_put(pagetable->parent->dev);
+out:
  	return (unmapped == size) ? 0 : -EINVAL;
  }
Asking because it is a *security issue* if we get this wrong:
1. Is there any measure benefit with this patch? I believe tlb invalidation doesn't contribute much to the unmap latency. 2. We at least should insert a full memory barrier before reading the ttbr0 register to ensure that everything we did prior to that is visible to smmu. But then I guess the cost of the full barrier would be similar to the tlb invalidation.

Because it could lead to security issues or other very hard to debug issues, I would prefer this optimization only if there is a significant measurable gain.

-Akhil.




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