iommufd: Report ATS not supported status via IOMMU_GET_HW_INFO

If the IOMMU driver reports that ATS is not supported for a device, set
the IOMMU_HW_CAP_PCI_ATS_NOT_SUPPORTED flag in the returned hardware
capabilities.

This uses a negative flag for UAPI compatibility. Existing userspace
assumes ATS is supported if no flag is present. This also ensures that
new userspace works correctly on both old and new kernels, where a
zero value implies ATS support.

When this flag is set, ATS cannot be used for the device. When it is
clear, ATS may be enabled when an appropriate HWPT is attached.

Reviewed-by: Samiullah Khawaja <skhawaja@google.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Shameer Kolothum <skolothumtho@nvidia.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>

Author: Shameer Kolothum

drivers/iommu/iommufd/device.c

@@ -1624,6 +1624,10 @@ int iommufd_get_hw_info(struct iommufd_ucmd *ucmd)
 	if (device_iommu_capable(idev->dev, IOMMU_CAP_DIRTY_TRACKING))
 		cmd->out_capabilities |= IOMMU_HW_CAP_DIRTY_TRACKING;
 
+	/* Report when ATS cannot be used for this device */
+	if (!device_iommu_capable(idev->dev, IOMMU_CAP_PCI_ATS_SUPPORTED))
+		cmd->out_capabilities |= IOMMU_HW_CAP_PCI_ATS_NOT_SUPPORTED;
+
 	cmd->out_max_pasid_log2 = 0;
 	/*
 	 * Currently, all iommu drivers enable PASID in the probe_device()

include/uapi/linux/iommufd.h

@@ -695,11 +695,15 @@ enum iommu_hw_info_type {
  * @IOMMU_HW_CAP_PCI_PASID_PRIV: Privileged Mode Supported, user ignores it
  *                               when the struct
  *                               iommu_hw_info::out_max_pasid_log2 is zero.
+ * @IOMMU_HW_CAP_PCI_ATS_NOT_SUPPORTED: ATS is not supported or cannot be used
+ *                                      on this device (absence implies ATS
+ *                                      may be enabled)
  */
 enum iommufd_hw_capabilities {
 	IOMMU_HW_CAP_DIRTY_TRACKING = 1 << 0,
 	IOMMU_HW_CAP_PCI_PASID_EXEC = 1 << 1,
 	IOMMU_HW_CAP_PCI_PASID_PRIV = 1 << 2,
+	IOMMU_HW_CAP_PCI_ATS_NOT_SUPPORTED = 1 << 3,
 };
 
 /**