Re: [PATCH v3 0/3] iommu/arm-smmu-v3: Tegra264 invalidation workaround

[Mostafa Saleh <smostafa@google.com>]

On Mon, Jun 01, 2026 at 10:48:42AM +0000, Ashish Mhetre wrote:
> Nvidia Tegra264 SMMUs are affected by an erratum where a TLB entry can
> survive an invalidation that races with concurrent traffic targeting
> the same entry. The hardware-recommended software workaround is to
> issue every CFGI/TLBI command (each followed by CMD_SYNC) twice. The
> second issue must execute only after the first issue's CMD_SYNC has
> completed, giving the sequence:

This seems quite intrusive, will the TLB entry survive if you push a
full invalidation instead?

Thanks,
Mostafa

> 
>     TLBI/CFGI ... CMD_SYNC TLBI/CFGI ... CMD_SYNC
> 
> ATC_INV is not affected and must not be doubled.
> 
> This series implements the workaround by hooking the duplication into
> arm_smmu_cmdq_issue_cmdlist(), the single chokepoint that every
> synchronous CMDQ submission flows through.
> 
> Patch 1 is a preparatory refactor that factors the existing batch
> force-sync conditions out of arm_smmu_cmdq_batch_add_cmd_p() into a
> new arm_smmu_cmdq_batch_force_sync() helper. No functional change.
> 
> Patch 2 detects affected instances using the existing
> "nvidia,tegra264-smmu" compatible string, exposes the condition via a
> new ARM_SMMU_OPT_TLBI_TWICE option bit, and adds a static-inline
> arm_smmu_cmd_needs_tlbi_twice() classifier in arm-smmu-v3.h so that
> both the in-tree CMDQ path and the iommufd VSMMU path can share a
> single predicate.
> 
> Patch 3 wires the workaround in. arm_smmu_cmdq_issue_cmdlist() becomes
> a thin wrapper that re-issues a synced cmdlist a second time when the
> first command needs doubling. The Tegra264 condition is added to
> arm_smmu_cmdq_batch_force_sync() so a full batch carrying CFGI/TLBI
> commands flushes with sync=true and is then doubled. The iommufd
> VSMMU path (arm_vsmmu_cache_invalidate()) is also taught to split the
> user-supplied batch at every "needs doubling" / "doesn't need
> doubling" transition via a new arm_vsmmu_can_batch_cmd() predicate,
> since that path can otherwise mix CFGI/TLBI with ATC_INV in a single
> submission.
> 

> The series is based on Jason Gunthorpe's "Remove SMMUv3
> struct arm_smmu_cmdq_ent" series [1], specifically commit 13428b0bf794
> ("iommu/arm-smmu-v3: Directly encode TLBI commands") which is the
> final patch of that series in linux-next.
> 
> [1] https://lore.kernel.org/all/177919957385.1012282.14787407041669291032.b4-ty@kernel.org/
> 
> Changes since v2:
> - Add new prep patch 1/3 that factors the existing force-sync
>   conditions into arm_smmu_cmdq_batch_force_sync() (from Nicolin).
> - Move arm_smmu_cmd_needs_tlbi_twice() to arm-smmu-v3.h as static
>   inline taking (smmu, cmd*) and folding in the option check.
> - Plug the Tegra264 condition into arm_smmu_cmdq_batch_force_sync()
>   instead of carrying a separate need_sync in batch_add_cmd_p().
> - Fix iommufd batching: arm_vsmmu_cache_invalidate() can mix
>   CFGI/TLBI with ATC_INV in one batch. Split at the boundary via a
>   new arm_vsmmu_can_batch_cmd() predicate.
> - Patch 2 wording: "next patch wires" -> "a subsequent change will
>   wire".
> 
> v2: https://lore.kernel.org/all/20260529140830.629738-1-amhetre@nvidia.com/
> 
> 
> Nicolin Chen (1):
>   iommu/arm-smmu-v3: Factor out CMDQ batch force-sync conditions
> 
> Ashish Mhetre (2):
>   iommu/arm-smmu-v3: Detect Tegra264 erratum
>   iommu/arm-smmu-v3: Issue CFGI/TLBI twice on Tegra264
> 
>  .../arm/arm-smmu-v3/arm-smmu-v3-iommufd.c     | 23 ++++++-
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c   | 66 +++++++++++++++----
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h   | 40 +++++++++++
>  3 files changed, 117 insertions(+), 12 deletions(-)
> 
> 
> base-commit: 13428b0bf7947098daf9a1db14a74d33eb1b5079
> -- 
> 2.50.1
> 
>