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drivers/iommu:riscv:add dma_wmb before cmpxchg_relaxed#309

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drivers/iommu:riscv:add dma_wmb before cmpxchg_relaxed#309
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RVCK-Project:rvck-6.6from
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@GooTal

@GooTal GooTal commented Jun 17, 2026 •
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drivers/iommu:riscv:add dma_wmb before cmpxchg_relaxed

driver inclusion
category: bugfix
bugzilla: #308

When multiple cores execute riscv_iommu_map_pages/riscv_iommu_pte_alloc, a parallel consistency problem can occur.
Setting dma_wmb ensures that all previous DMA write operations have been completed before cmpxchg_relaxed.

Signed-off-by: bailu bai.lu5@zte.com.cn
Signed-off-by: liuqingtao liu.qingtao2@zte.com.cn

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github-actions Bot commented Jun 17, 2026 •
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开始测试 log: https://github.com/RVCK-Project/rvck/actions/runs/27664081160

参数解析结果
args value
repository RVCK-Project/rvck
head ref pull/309/head
base ref rvck-6.6
LAVA repo RVCK-Project/lavaci
LAVA hardware ['qemu']
LAVA Testcase path lava-testcases/common-test/ltp/ltp.yaml
need run job kunit-test,kernel-build,check-patch,lava-trigger

测试完成

详细结果:
check result
kunit-test success
kernel-build success
check-patch failure
lava-trigger-qemu success
lava-trigger-sg2042 skipped
lava-trigger-k1 skipped
lava-trigger-lpi4a skipped

Kunit Test Result

[03:42:15] Testing complete. Ran 482 tests: passed: 466, skipped: 16

Kernel Build Result

Check Patch Result
Total Errors 1
Total Warnings 1

LAVA Check (qemu)

args value
testcase_repo RVCK-Project/lavaci
lava_template lava-job-template/qemu/qemu-ltp.yaml
testcase_path lava-testcases/common-test/ltp/ltp.yaml
kernel_download_url http://10.30.190.110/openEuler-RISC-V/RVCK/OERV-RVCI/RVCK-Project/rvck/309_27664081160_1/Image
initramfs_download_url http://10.30.190.110/openEuler-RISC-V/RVCK/OERV-RVCI/RVCK-Project/rvck/309_27664081160_1/initramfs.img
rootfs_download_url https://fast-mirror.isrc.ac.cn/openeuler-sig-riscv/openEuler-RISC-V/RVCK/openEuler24.03-LTS-SP1/openeuler-rootfs.img.zst
testcase_ref main
testitem_name RVCK-Project_rvck_pull_request_target_309__common-test_qemu

result: Lava check done!

@GooTal GooTal force-pushed the pr-iommu-dma_wmb branch from 72cde4e to b42cb3f Compare June 17, 2026 08:54
@github-actions

github-actions Bot commented Jun 17, 2026 •
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开始测试 log: https://github.com/RVCK-Project/rvck/actions/runs/27677458532

参数解析结果
args value
repository RVCK-Project/rvck
head ref pull/309/head
base ref rvck-6.6
LAVA repo RVCK-Project/lavaci
LAVA hardware ['qemu']
LAVA Testcase path lava-testcases/common-test/ltp/ltp.yaml
need run job kunit-test,kernel-build,check-patch,lava-trigger

测试完成

详细结果:
check result
kunit-test success
kernel-build success
check-patch success
lava-trigger-qemu success
lava-trigger-sg2042 skipped
lava-trigger-k1 skipped
lava-trigger-lpi4a skipped

Kunit Test Result

[08:58:23] Testing complete. Ran 482 tests: passed: 466, skipped: 16

Kernel Build Result

Check Patch Result
Total Errors 0
Total Warnings 0

LAVA Check (qemu)

args value
testcase_repo RVCK-Project/lavaci
lava_template lava-job-template/qemu/qemu-ltp.yaml
testcase_path lava-testcases/common-test/ltp/ltp.yaml
kernel_download_url http://10.30.190.110/openEuler-RISC-V/RVCK/OERV-RVCI/RVCK-Project/rvck/309_27677458532_1/Image
initramfs_download_url http://10.30.190.110/openEuler-RISC-V/RVCK/OERV-RVCI/RVCK-Project/rvck/309_27677458532_1/initramfs.img
rootfs_download_url https://fast-mirror.isrc.ac.cn/openeuler-sig-riscv/openEuler-RISC-V/RVCK/openEuler24.03-LTS-SP1/openeuler-rootfs.img.zst
testcase_ref main
testitem_name RVCK-Project_rvck_pull_request_target_309__common-test_qemu

result: Lava check done!

@fangyu0809

fangyu0809 commented Jun 18, 2026 •
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The real race here is CPU-CPU (concurrent pte_alloc callers), not CPU-to-DMA-device. dma_wmb fixes it functionally but is semantically misleading. Suggest using cmpxchg_release instead,— matching upstream generic_pt (pt_table_install64 uses try_cmpxchg64_release)

  • ret = cmpxchg_relaxed(ptep, 0ULL, pte);
  • ret = cmpxchg_release(ptep, 0ULL, pte);

Also, the CPU vs IOMMU-HW-walker race likely doesn't exist for new PDE installation,Please double-check whether this race is actually reachable in practice.

@king729

king729 commented Jun 22, 2026

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@fangyu0809
The trade-off of this modification come from the current ARM implementation:

/*
 * Ensure the table itself is visible before its PTE can be.
 * Whilst we could get away with cmpxchg64_release below, this
 * doesn't have any ordering semantics when !CONFIG_SMP.
 */
dma_wmb();

old = cmpxchg64_relaxed(ptep, curr, new);

As the comment explains, the introduction of dma_wmb() primarily addresses the lack of ordering guarantees for cmpxchg64_release on non‑SMP architectures.

The new version modifies this to:

if (!IS_ENABLED(CONFIG_SMP))
    dma_wmb();
ret = try_cmpxchg64_release(entryp, &old_entry, table_entry);

I believe this adjustment is also intended to maintain compatibility across both SMP and non‑SMP configurations.

Therefore, in my current modification, I have chosen to adopt the earlier approach — using dma_wmb() combined with cmpxchg64_relaxed — for consistency and to preserve the original ordering semantics.

@GooTal
drivers/iommu:riscv:add dma_wmb before cmpxchg_relaxed
26f0782 
driver inclusion
category: bugfix
Link: RVCK-Project#308

--------------------------------

When multiple cores execute riscv_iommu_map_pages/riscv_iommu_pte_alloc,
a parallel consistency problem can occur.
Setting dma_wmb ensures that all previous DMA write operations have
been completed before cmpxchg_relaxed.

Signed-off-by: bailu <bai.lu5@zte.com.cn>
Signed-off-by: liuqingtao <liu.qingtao2@zte.com.cn>
@GooTal GooTal force-pushed the pr-iommu-dma_wmb branch from b42cb3f to 26f0782 Compare June 26, 2026 01:02
@github-actions

github-actions Bot commented Jun 26, 2026 •
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开始测试 log: https://github.com/RVCK-Project/rvck/actions/runs/28210386797

参数解析结果
args value
repository RVCK-Project/rvck
head ref pull/309/head
base ref rvck-6.6
LAVA repo RVCK-Project/lavaci
LAVA hardware ['qemu']
LAVA Testcase path lava-testcases/common-test/ltp/ltp.yaml
need run job kunit-test,kernel-build,check-patch,lava-trigger

测试完成

详细结果:
check result
kunit-test success
kernel-build success
check-patch failure
lava-trigger-qemu success
lava-trigger-sg2042 skipped
lava-trigger-k1 skipped
lava-trigger-lpi4a skipped

Kunit Test Result

[01:07:04] Testing complete. Ran 482 tests: passed: 466, skipped: 16

Kernel Build Result

Check Patch Result
Total Errors 20
Total Warnings 1

LAVA Check (qemu)

args value
testcase_repo RVCK-Project/lavaci
lava_template lava-job-template/qemu/qemu-ltp.yaml
testcase_path lava-testcases/common-test/ltp/ltp.yaml
kernel_download_url http://10.30.190.110/openEuler-RISC-V/RVCK/OERV-RVCI/RVCK-Project/rvck/309_28210386797_1/Image
initramfs_download_url http://10.30.190.110/openEuler-RISC-V/RVCK/OERV-RVCI/RVCK-Project/rvck/309_28210386797_1/initramfs.img
rootfs_download_url https://fast-mirror.isrc.ac.cn/openeuler-sig-riscv/openEuler-RISC-V/RVCK/openEuler24.03-LTS-SP1/openeuler-rootfs.img.zst
testcase_ref main
testitem_name RVCK-Project_rvck_pull_request_target_309__common-test_qemu

result: Lava check done!

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4 participants

@GooTal @fangyu0809 @king729 @sterling-teng