linux-kernel - [PATCH rc v4 0/4] iommu/arm-smmu-v3: Fix hitless STE update in nesting cases

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Message-ID: <cover.1765945258.git.nicolinc@nvidia.com>
Date: Tue, 16 Dec 2025 20:25:58 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: <jgg@...dia.com>, <will@...nel.org>, <robin.murphy@....com>
CC: <smostafa@...gle.com>, <joro@...tes.org>,
	<linux-arm-kernel@...ts.infradead.org>, <iommu@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>, <skolothumtho@...dia.com>,
	<praan@...gle.com>, <xueshuai@...ux.alibaba.com>
Subject: [PATCH rc v4 0/4] iommu/arm-smmu-v3: Fix hitless STE update in nesting cases

Occasional C_BAD_STE errors were observed in nesting setups where a device
attached to a nested bypass/identity domain enables PASID.

This occurred when the physical STE was updated from S2-only mode to S1+S2
nesting mode, but the update failed to use the hitless routine that it was
supposed to use. Instead, it cleared STE.V bit to load the CD table, while
the default substream was still actively performing DMA.

It was later found that the diff algorithm in arm_smmu_entry_qword_diff()
enforced an additional critical word due to misaligned MEV and EATS fields
between S2-only and S1+S2 modes.

Both fields are either well-managed or non-critical, so move them to the
"ignored" list to relax the qword diff algorithm.

Additionally, add KUnit test coverage for these nesting STE cases.

This is on Github:
https://github.com/nicolinc/iommufd/commits/smmuv3_ste_fixes/

A host kernel must apply this to fix the bug.

Changelog
v4:
 * s/ignored/update_safe
 * Change entry_set to void
v3:
 https://lore.kernel.org/all/cover.1765334526.git.nicolinc@nvidia.com/
 * Add Reviewed-by from Shuai
 * Add an inline comments in nested test cases
 * Reuse arm_smmu_test_make_cdtable_ste() for nested test cases
v2:
 https://lore.kernel.org/all/cover.1765140287.git.nicolinc@nvidia.com/
 * Fix kunit tests
 * Update commit message and inline comments
 * Keep MEV/EATS in used list by masking them away using ignored_bits
v1:
 https://lore.kernel.org/all/cover.1764982046.git.nicolinc@nvidia.com/

Jason Gunthorpe (3):
  iommu/arm-smmu-v3: Add update_safe bits to fix STE update sequence
  iommu/arm-smmu-v3: Mark STE MEV safe when computing the update
    sequence
  iommu/arm-smmu-v3: Mark STE EATS safe when computing the update
    sequence

Nicolin Chen (1):
  iommu/arm-smmu-v3-test: Add nested s1bypass/s1dssbypass coverage

 drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h   |  2 +
 .../iommu/arm/arm-smmu-v3/arm-smmu-v3-test.c  | 64 ++++++++++++++++++-
 drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c   | 46 +++++++++++--
 3 files changed, 102 insertions(+), 10 deletions(-)

-- 
2.43.0