Re: [PATCH rc v5 2/4] iommu/arm-smmu-v3: Mark STE MEV safe when computing the update sequence

[Mostafa Saleh <smostafa@google.com>]

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 <nicolinc@nvidia.com>

Reviewed-by: Mostafa Saleh <smostafa@google.com>

Thanks,
Mostafa

> ---
>  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
>