linux-kernel - [PATCH 1/1] iommu/vt-d: PRS isn't usable if PDS isn't supported

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Message-ID: <20250915062946.120196-1-baolu.lu@linux.intel.com>
Date: Mon, 15 Sep 2025 14:29:46 +0800
From: Lu Baolu <baolu.lu@...ux.intel.com>
To: Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>,
	Kevin Tian <kevin.tian@...el.com>,
	Jason Gunthorpe <jgg@...dia.com>,
	Joel Granados <joel.granados@...nel.org>
Cc: iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	Lu Baolu <baolu.lu@...ux.intel.com>,
	stable@...r.kernel.org
Subject: [PATCH 1/1] iommu/vt-d: PRS isn't usable if PDS isn't supported

The specification, Section 7.10, "Software Steps to Drain Page Requests &
Responses," requires software to submit an Invalidation Wait Descriptor
(inv_wait_dsc) with the Page-request Drain (PD=1) flag set, along with
the Invalidation Wait Completion Status Write flag (SW=1). It then waits
for the Invalidation Wait Descriptor's completion.

However, the PD field in the Invalidation Wait Descriptor is optional, as
stated in Section 6.5.2.9, "Invalidation Wait Descriptor":

"Page-request Drain (PD): Remapping hardware implementations reporting
 Page-request draining as not supported (PDS = 0 in ECAP_REG) treat this
 field as reserved."

This implies that if the IOMMU doesn't support the PDS capability, software
can't drain page requests and group responses as expected.

Do not enable PCI/PRI if the IOMMU doesn't support PDS.

Reported-by: Joel Granados <joel.granados@...nel.org>
Closes: https://lore.kernel.org/r/20250909-jag-pds-v1-1-ad8cba0e494e@kernel.org
Fixes: 66ac4db36f4c ("iommu/vt-d: Add page request draining support")
Cc: stable@...r.kernel.org
Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
---
 drivers/iommu/intel/iommu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
index 9c3ab9d9f69a..92759a8f8330 100644
--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -3812,7 +3812,7 @@ static struct iommu_device *intel_iommu_probe_device(struct device *dev)
 			}
 
 			if (info->ats_supported && ecap_prs(iommu->ecap) &&
-			    pci_pri_supported(pdev))
+			    ecap_pds(iommu->ecap) && pci_pri_supported(pdev))
 				info->pri_supported = 1;
 		}
 	}
-- 
2.43.0