RE: [PATCH 2/2] iommu/vt-d: Remove hardware automatic ATS dependency

Tian, Kevin
2024-06-25T02:32:07Z

> From: Lu Baolu <baolu.lu@linux.intel.com>
> Sent: Monday, June 24, 2024 1:25 PM
>
> If a device is listed in the SATC table with ATC_REQUIRED flag set, it
> indicates that the device has a functional requirement to enable its ATC
> (via the ATS capability) for device operation. However, when IOMMU is
> running in the legacy mode, ATS could be automatically supported by the
> hardware so that the OS has no need to support the ATS functionality.

hmm I don't think "has no need to support" matches...

>
> This is a backward compatibility feature which enables older OSs. Since
> Linux VT-d implementation has already supported ATS features for a long
> time, there is no need to rely on this compatibility hardware. Remove it
> to make the driver future-proof.
>
> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
> ---
> drivers/iommu/intel/iommu.c | 9 +--------
> 1 file changed, 1 insertion(+), 8 deletions(-)
>
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 07e394dfccc1..b63347c8bf5d 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -3056,14 +3056,7 @@ static bool dmar_ats_supported(struct pci_dev
> *dev, struct intel_iommu *iommu)
> 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.
> - */

...what above comment tries to convey.

If this comment is valid, it's not about whether the OS itself supports
ATS. instead it's a requirement for the OS to not manage ATS when
it's already managed by HW.

Unless there is a way to disable hw management with this change...

> - return !(satcu->atc_required && !sm_supported(iommu));
> + return true;
>
> for (bus = dev->bus; bus; bus = bus->parent) {
> bridge = bus->self;
> --
> 2.34.1