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

[Akhil P Oommen]

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.