Re: [PATCH rc v2 2/4] iommu/arm-smmu-v3: Ignore STE MEV when computing the update sequence

[Shuai Xue]

在 2025/12/8 04:49, Nicolin Chen 写道:
> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
>
> 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 ignore MEV 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@xxxxxxxxxxxxxxx
> Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Signed-off-by: Nicolin Chen <nicolinc@xxxxxxxxxx>
> ---
>   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 e22c0890041b..3e161d8298d9 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_ignored(__le64 *ignored_bits)
>   {
> +	/*
> +	 * MEV does not meaningfully impact the operation of the HW, it only
> +	 * changes how many fault events are generated, thus we can ignore 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.
> +	 */
> +	ignored_bits[1] |= cpu_to_le64(STRTAB_STE_1_MEV);
>   }
>   EXPORT_SYMBOL_IF_KUNIT(arm_smmu_get_ste_ignored);
>   


Reviewed-by: Shuai Xue <xueshuai@xxxxxxxxxxxxxxxxx>

Thanks.
Shuai