Re: [PATCH v1 3/3] iommu/vt-d: Fix ATS support check for integrated endpoints

[Yi Liu <yi.l.liu@intel.com>]

On 2025/5/9 22:00, Wei Wang wrote:
> The VT-d spec states that "SATC reporting structure identifies devices
> that have address translation cache and that is validated per requirements
> described in the 'Device TLB in System-on-Chip (SoC) Integrated Devices'
> section. It is recommended that system software enable ATC for this
> device". It is possible for an integrated device to have PCI ATC
> capability implemented but not validated per the requirements, and thus
> not appear in the SATC structure as recommended for ATS enablement.
> 
> The current implementation checks ATS support for integrated endpoints in
> two places. First, it verifies if the integrated endpoint device is listed
> in SATC. If not, it proceeds to the second check that always returns true
> for integrated devices. This could result in endpoint devices not
> recommended in SATC presenting "supported = true" to the caller.
> 
> Add integrated_device_ats_supported() for the integrated device ATS check
> in a single location, which improves readability. The above issue is
> also fixed in the function via returning false in that case.

if it is a fix. A Fixes tag is needed.

> Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> ---
>   drivers/iommu/intel/iommu.c | 42 +++++++++++++++++++++++++------------
>   1 file changed, 29 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 2778bfe14f36..39abcf4e0f8f 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -2760,6 +2760,34 @@ static struct dmar_satc_unit *dmar_find_matched_satc_unit(struct pci_dev *dev)
>   	return satcu;
>   }
>   
> +static bool integrated_device_ats_supported(struct pci_dev *dev, struct intel_iommu *iommu)
> +{
> +	struct dmar_satc_unit *satcu = dmar_find_matched_satc_unit(dev);
> +
> +	/*
> +	 * This device supports ATS as it is in SATC table. When IOMMU is in
> +	 * legacy mode, enabling ATS is done automatically by HW for the device
> +	 * that requires ATS, hence OS should not enable this device ATS to
> +	 * avoid duplicated TLB invalidation.
> +	 */
> +	if (satcu)
> +		return !(satcu->atc_required && !sm_supported(iommu));
> +
> +	/*
> +	 * The integrated device isn't enumerated in the SATC structure. For
> +	 * example, it has ATS PCI capability implemented but not validated per
> +	 * the requirements described in the VT-d specification, specifically
> +	 * in the "Device TLB in System-on-Chip (SoC) Integrated Devices"
> +	 * section. Therefore, it does not appear in the SATC structure. Return
> +	 * false in this case.
> +	 *
> +	 * On older machines that do not support SATC (i.e., no SATC structure
> +	 * present), ATS is considered to be "always" supported for integrated
> +	 * endpoints.
> +	 */
> +	return !list_empty(&dmar_satc_units);

shouldn't it be "return list_empty(&dmar_satc_units);"?

> +}
> +
>   static bool dmar_ats_supported(struct pci_dev *dev, struct intel_iommu *iommu)
>   {
>   	int i;
> @@ -2769,25 +2797,13 @@ static bool dmar_ats_supported(struct pci_dev *dev, struct intel_iommu *iommu)
>   	struct device *tmp;
>   	struct acpi_dmar_atsr *atsr;
>   	struct dmar_atsr_unit *atsru;
> -	struct dmar_satc_unit *satcu;
>   
>   	dev = pci_physfn(dev);
> -	satcu = dmar_find_matched_satc_unit(dev);
> -	if (satcu)
> -		/*
> -		 * This device supports ATS as it is in SATC table.
> -		 * When IOMMU is in legacy mode, enabling ATS is done
> -		 * automatically by HW for the device that requires
> -		 * ATS, hence OS should not enable this device ATS
> -		 * to avoid duplicated TLB invalidation.
> -		 */
> -		return !(satcu->atc_required && !sm_supported(iommu));
>   
>   	for (bus = dev->bus; bus; bus = bus->parent) {
>   		bridge = bus->self;
> -		/* If it's an integrated device, allow ATS */
>   		if (!bridge)
> -			return true;
> +			return integrated_device_ats_supported(dev, iommu);
>   		/* Connected via non-PCIe: no ATS */
>   		if (!pci_is_pcie(bridge) ||
>   		    pci_pcie_type(bridge) == PCI_EXP_TYPE_PCI_BRIDGE)

-- 
Regards,
Yi Liu