iommu/vt-d: PRS isn't usable if PDS isn't supported

PATCH In review Lu Baolu 2 participants 5 messages Sep 15, 2025 – Sep 19, 2025

Subsystems (3)

Stable branch IOMMU Intel IOMMU (VT-d)

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Lu Baolu

author maintainer

2025-09-15 06:32 UTC #0 lore raw

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 <[email protected]>

Closes https://lore.kernel.org/r/[email protected] open here

Fixes 66ac4db36f4c ("iommu/vt-d: Add page request draining support")

Cc [email protected]

Signed-off-by Lu Baolu <[email protected]>

---
 drivers/iommu/intel/iommu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

drivers/iommu/intel/iommu.c +1 -1

index 9c3ab9d9f69a..92759a8f8330 100644
@@ -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;
 		}
 	}

Signature · 1 line

2.43.0

Joel Granados

2025-09-15 11:31 UTC #1 lore raw

Quoted 9 lines · Lu Baolu

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":

Quoted 8 lines

"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.

After giving the spec another look, this is probably the way to go. However the PDS also mentions that DT must be set. Should we check ecap_dev_iotlb_support(iommu->ecap) as well?

Quoted 15 lines

Reported-by Joel Granados <[email protected]>
Closes https://lore.kernel.org/r/[email protected] open here
Fixes 66ac4db36f4c ("iommu/vt-d: Add page request draining support")
Cc [email protected]
Signed-off-by Lu Baolu <[email protected]>
---
 drivers/iommu/intel/iommu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
drivers/iommu/intel/iommu.c +0 -0
index 9c3ab9d9f69a..92759a8f8330 100644

drivers/iommu/intel/iommu.c static struct iommu_device *intel_iommu_probe_device(struct device *dev) · line 3815

Quoted 5 lines

drivers/iommu/intel/iommu.c +1 -1

@@ -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))

Should this be
+ ecap_dev_iotlb_support(iommu->ecap) && ecap_pds(iommu->ecap) && pci_pri_supported(pdev))

???

best

Signature · 1 line

Joel Granados

Baolu Lu

author maintainer

2025-09-16 02:01 UTC #2 lore raw

Quoted 14 lines · Joel Granados

Quoted 13 lines · Lu Baolu
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."

Quoted 8 lines

Quoted 4 lines
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.
After giving the spec another look, this is probably the way to go. However the PDS also mentions that DT must be set. Should we check ecap_dev_iotlb_support(iommu->ecap) as well?

It has already been checked.

Quoted 17 lines

Quoted 17 lines
Reported-by Joel Granados <[email protected]>
Closes https://lore.kernel.org/r/[email protected] open here
Fixes 66ac4db36f4c ("iommu/vt-d: Add page request draining support")
Cc [email protected]
Signed-off-by Lu Baolu <[email protected]>
---
  drivers/iommu/intel/iommu.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)
drivers/iommu/intel/iommu.c +0 -0
index 9c3ab9d9f69a..92759a8f8330 100644
@@ -3812,7 +3812,7 @@ static struct iommu_device *intel_iommu_probe_device(struct device *dev)
  			}
 

Quoted 7 lines

drivers/iommu/intel/iommu.c static struct iommu_device *intel_iommu_probe_device(struct device *dev) · line 3815
Quoted 3 lines
drivers/iommu/intel/iommu.c +1 -1
  			if (info->ats_supported && ecap_prs(iommu->ecap) &&
-			    pci_pri_supported(pdev))
+			    ecap_pds(iommu->ecap) && pci_pri_supported(pdev))
Should this be
+ ecap_dev_iotlb_support(iommu->ecap) && ecap_pds(iommu->ecap) && pci_pri_supported(pdev))
???

ecap_dev_iotlb_support() has already been checked as it's part of the info->ats_supported check. The dependency chain is:
ecap_dev_iotlb_support() -> ats_support -> pri_support.

Thanks,
baolu

Joel Granados

2025-09-19 07:13 UTC #3 lore raw

Quoted 18 lines · Baolu Lu

Quoted 17 lines · Joel Granados
Quoted 16 lines · Lu Baolu
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.

Quoted 7 lines

Quoted 5 lines
Quoted 1 line
Do not enable PCI/PRI if the IOMMU doesn't support PDS.
After giving the spec another look, this is probably the way to go. However the PDS also mentions that DT must be set. Should we check ecap_dev_iotlb_support(iommu->ecap) as well?
It has already been checked.

Yes. In intel_iommu_probe_device. Thx for the clarification.

Best

Signature · 1 line

Joel Granados

Baolu Lu

author maintainer

2025-09-18 04:38 UTC #4 lore raw

In reply to Lu Baolu

Quoted 18 lines · Lu Baolu

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.

Quoted 8 lines

Reported-by Joel Granados<[email protected]>
Closes https://lore.kernel.org/r/[email protected] open here
Fixes 66ac4db36f4c ("iommu/vt-d: Add page request draining support")
Cc [email protected]
Signed-off-by Lu Baolu<[email protected]>
---
  drivers/iommu/intel/iommu.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

Queued for v6.18.