Re: [PATCH 5/5] drm/msm: Skip tlbinv on unmap from non-current pgtables
From: Akhil P Oommen
Date: Thu Aug 25 2022 - 14:12:45 EST
On 8/25/2022 12:32 AM, Rob Clark wrote:
On Wed, Aug 24, 2022 at 10:46 AM Akhil P Oommen
<quic_akhilpo@xxxxxxxxxxx> wrote:
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.
It turned out to not make a huge difference.. although I expect the
part about skipping the inv when runtime suspended is still useful
from a power standpoint (but don't have a great setup to measure that)
Agree. Perhaps use the recently added 'suspended' flag instead of
pm_runtime_get_if_in_use().
-Akhil.
BR,
-R
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.