Re: [PATCH 1/1] iommu/vt-d: Prevent boot failure with devices requiring ATS

[Mark Pearson]

Hi Lu,

Tested this on an X1 Carbon G12 with a kernel built form drm-tip and this patch - and was able to boot successfully with pci=noats

Tested-by: Mark Pearson <mpearson-lenovo@xxxxxxxxx>

Mark

PS - note on small typo below.

On Wed, Sep 4, 2024, at 2:07 AM, Lu Baolu wrote:
> SOC-integrated devices on some platforms require their PCI ATS enabled
> for operation when the IOMMU is in scalable mode. Those devices are
> reported via ACPI/SATC table with the ATC_REQUIRED bit set in the Flags
> field.
>
> The PCI subsystem offers the 'pci=noats' kernel command to disable PCI
> ATS on all devices. Using 'pci=noat' with devices that require PCI ATS

pci=noats

> can cause a conflict, leading to boot failure, especially if the device
> is a graphics device.
>
> To prevent this issue, check PCI ATS support before enumerating the IOMMU
> devices. If any device requires PCI ATS, but PCI ATS is disabled by
> 'pci=noats', switch the IOMMU to operate in legacy mode to ensure
> successful booting.
>
> Fixes: 97f2f2c5317f ("iommu/vt-d: Enable ATS for the devices in SATC table")
> Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/12036
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> ---
>  drivers/iommu/intel/iommu.c | 22 +++++++++++++++++++---
>  1 file changed, 19 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 4aa070cf56e7..8f275e046e91 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -3127,10 +3127,26 @@ int dmar_iommu_notify_scope_dev(struct 
> dmar_pci_notify_info *info)
>  					(void *)satc + satc->header.length,
>  					satc->segment, satcu->devices,
>  					satcu->devices_cnt);
> -			if (ret > 0)
> -				break;
> -			else if (ret < 0)
> +			if (ret < 0)
>  				return ret;
> +
> +			if (ret > 0) {
> +				/*
> +				 * The device requires PCI/ATS when the IOMMU
> +				 * works in the scalable mode. If PCI/ATS is
> +				 * disabled using the pci=noats kernel parameter,
> +				 * the IOMMU will default to legacy mode. Users
> +				 * are informed of this change.
> +				 */
> +				if (intel_iommu_sm && satcu->atc_required &&
> +				    !pci_ats_supported(info->dev)) {
> +					pci_warn(info->dev,
> +						 "PCI/ATS not supported, system working in IOMMU legacy mode\n");
> +					intel_iommu_sm = 0;
> +				}
> +
> +				break;
> +			}
>  		} else if (info->event == BUS_NOTIFY_REMOVED_DEVICE) {
>  			if (dmar_remove_dev_scope(info, satc->segment,
>  					satcu->devices, satcu->devices_cnt))
> -- 
> 2.34.1