Re: [PATCH rc] iommu/arm-smmu-v3: Do not set disable_ats unless vSTE is Translate

[Pranjal Shrivastava <praan@google.com>]

On Wed, Jan 14, 2026 at 05:12:43PM -0800, Nicolin Chen wrote:
> A vSTE may have three configuration types: Abort, Bypass, and Translate.
> 
> An Abort vSTE wouldn't enable ATS, but the other two might.
> 
> It makes sense for a Transalte vSTE to rely on the guest vSTE.EATS field.
> 
> For a Bypass vSTE, it would end up with an S2-only physical STE, similar
> to an attachment to a regular S2 domain. However, the nested case always
> disables ATS following the Bypass vSTE, while the regular S2 case always
> enables ATS so long as arm_smmu_ats_supported(master) == true.
> 
> Note that ATS is needed for certain VM centric workloads and historically
> non-vSMMU cases have relied on this automatic enablement. So, having the
> nested case behave differently causes problems.
> 
> To fix that, add a condition to disable_ats, so that it might enable ATS
> for a Bypass vSTE, aligning with the regular S2 case.
> 
> Fixes: f27298a82ba0 ("iommu/arm-smmu-v3: Allow ATS for IOMMU_DOMAIN_NESTED")
> Cc: stable@vger.kernel.org
> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
> ---
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c
> index 93fdadd07431..823461a26659 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c
> @@ -177,7 +177,9 @@ static int arm_smmu_attach_dev_nested(struct iommu_domain *domain,
>  	 * config bit here base this off the EATS value in the STE. If the EATS
>  	 * is set then the VM must generate ATC flushes.
>  	 */
> -	state.disable_ats = !nested_domain->enable_ats;
> +	if (FIELD_GET(STRTAB_STE_0_CFG, le64_to_cpu(nested_domain->ste[0])) ==
> +	    STRTAB_STE_0_CFG_S1_TRANS)
> +		state.disable_ats = !nested_domain->enable_ats;
>  	ret = arm_smmu_attach_prepare(&state, domain);
>  	if (ret) {
>  		mutex_unlock(&arm_smmu_asid_lock);

This makes sense. The nested_domain->enable_ats should indeed only be
checked for Translate configs.

Reviewed-by: Pranjal Shrivastava <praan@google.com>

Thanks,
Praan