Re: [PATCH 1/2] iommu: Add device ATS not supported capability

[Samiullah Khawaja]

On Tue, Mar 03, 2026 at 03:03:47PM +0000, Shameer Kolothum wrote:
> PCIe ATS may be disabled by platform firmware, root complex limitations,
> or kernel policy even when a device advertises the ATS capability in its
> PCI configuration space.
>
> Add a new IOMMU_CAP_PCI_ATS_NOT_SUPPORTED capability to allow IOMMU
> drivers to report the effective ATS decision for a device.

nit: Maybe have IOMMU_CAP_PCI_ATS_SUPPORTED instead of the negative
variant of the capability. As this will affect only new users who might
check this capability so this does not really have to be negative.
> When this capability is returned true for a device, ATS is not
> supported and not used for that device, regardless of the presence
> of the PCI ATS capability.
>
> A subsequent patch will extend iommufd to expose the effective ATS
> status to userspace.
>
> Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Signed-off-by: Shameer Kolothum <skolothumtho@xxxxxxxxxx>
> ---
> include/linux/iommu.h                       | 2 ++
> drivers/iommu/amd/iommu.c                   | 6 ++++++
> drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 3 +++
> drivers/iommu/intel/iommu.c                 | 2 ++
> 4 files changed, 13 insertions(+)
>
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index 54b8b48c762e..f40ecdc5d761 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -271,6 +271,8 @@ enum iommu_cap {
> 	 */
> 	IOMMU_CAP_DEFERRED_FLUSH,
> 	IOMMU_CAP_DIRTY_TRACKING,	/* IOMMU supports dirty tracking */
> +	/* ATS is not supported and not used on this device */
> +	IOMMU_CAP_PCI_ATS_NOT_SUPPORTED,
> };
>
> /* These are the possible reserved region types */
> diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
> index 81c4d7733872..aa4399b6b6db 100644
> --- a/drivers/iommu/amd/iommu.c
> +++ b/drivers/iommu/amd/iommu.c
> @@ -2985,6 +2985,12 @@ static bool amd_iommu_capable(struct device *dev, enum iommu_cap cap)
>
> 		return amd_iommu_hd_support(iommu);
> 	}
> +	case IOMMU_CAP_PCI_ATS_NOT_SUPPORTED: {
> +		struct iommu_dev_data *dev_data = dev_iommu_priv_get(dev);
> +
> +		return !(amd_iommu_iotlb_sup &&
> +			 (dev_data->flags & AMD_IOMMU_DEVICE_FLAG_ATS_SUP));
> +	}
> 	default:
> 		break;
> 	}
> 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 4d00d796f078..c20d2454ca14 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> @@ -107,6 +107,7 @@ static const char * const event_class_str[] = {
> };
>
> static int arm_smmu_alloc_cd_tables(struct arm_smmu_master *master);
> +static bool arm_smmu_ats_supported(struct arm_smmu_master *master);
>
> static void parse_driver_options(struct arm_smmu_device *smmu)
> {
> @@ -2494,6 +2495,8 @@ static bool arm_smmu_capable(struct device *dev, enum iommu_cap cap)
> 		return true;
> 	case IOMMU_CAP_DIRTY_TRACKING:
> 		return arm_smmu_dbm_capable(master->smmu);
> +	case IOMMU_CAP_PCI_ATS_NOT_SUPPORTED:
> +		return !arm_smmu_ats_supported(master);
> 	default:
> 		return false;
> 	}
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index ef7613b177b9..0be69695e88a 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -3220,6 +3220,8 @@ static bool intel_iommu_capable(struct device *dev, enum iommu_cap cap)
> 		return ecap_sc_support(info->iommu->ecap);
> 	case IOMMU_CAP_DIRTY_TRACKING:
> 		return ssads_supported(info->iommu);
> +	case IOMMU_CAP_PCI_ATS_NOT_SUPPORTED:
> +		return !info->ats_supported;
> 	default:
> 		return false;
> 	}
> --
> 2.43.0

Thanks,
Sami