Re: [PATCH rc v5 2/4] iommu/arm-smmu-v3: Mark STE MEV safe when computing the update sequence
From: Mostafa Saleh <smostafa@google.com>
Date: 2026-01-02 18:27:11
Also in:
linux-iommu, lkml
On Thu, Dec 18, 2025 at 01:41:57PM -0800, Nicolin Chen wrote:
From: Jason Gunthorpe <jgg@nvidia.com>
Nested CD tables set the MEV bit to try to reduce multi-fault spamming on
the hypervisor. Since MEV is in STE word 1 this causes a breaking update
sequence that is not required and impacts real workloads.
For the purposes of STE updates the value of MEV doesn't matter, if it is
set/cleared early or late it just results in a change to the fault reports
that must be supported by the kernel anyhow. The spec says:
Note: Software must expect, and be able to deal with, coalesced fault
records even when MEV == 0.
So mark STE MEV safe when computing the update sequence, to avoid creating
a breaking update.
Fixes: da0c56520e88 ("iommu/arm-smmu-v3: Set MEV bit in nested STE for DoS mitigations")
Cc: stable@vger.kernel.org
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Shuai Xue <xueshuai@linux.alibaba.com>
Signed-off-by: Nicolin Chen <redacted>Reviewed-by: Mostafa Saleh <smostafa@google.com> Thanks, Mostafa
quoted hunk ↗ jump to hunk
--- drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 10 ++++++++++ 1 file changed, 10 insertions(+)diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c index 8dbf4ad5b51e..12a9669bcc83 100644 --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c@@ -1085,6 +1085,16 @@ EXPORT_SYMBOL_IF_KUNIT(arm_smmu_get_ste_used); VISIBLE_IF_KUNIT void arm_smmu_get_ste_update_safe(__le64 *safe_bits) { + /* + * MEV does not meaningfully impact the operation of the HW, it only + * changes how many fault events are generated, thus we can relax it + * when computing the ordering. The spec notes the device can act like + * MEV=1 anyhow: + * + * Note: Software must expect, and be able to deal with, coalesced + * fault records even when MEV == 0. + */ + safe_bits[1] |= cpu_to_le64(STRTAB_STE_1_MEV); } EXPORT_SYMBOL_IF_KUNIT(arm_smmu_get_ste_update_safe);-- 2.43.0