linux-kernel - [PATCH 1/1] iommu/vt-d: Disallow dirty tracking if incoherent page walk

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Message-ID: <20250924083447.123224-1-baolu.lu@linux.intel.com>
Date: Wed, 24 Sep 2025 16:34:46 +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>
Cc: iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	Lu Baolu <baolu.lu@...ux.intel.com>
Subject: [PATCH 1/1] iommu/vt-d: Disallow dirty tracking if incoherent page walk

Dirty page tracking relies on the IOMMU atomically updating the dirty bit
in the paging-structure entry. For this operation to succeed, the paging-
structure memory must be coherent between the IOMMU and the CPU. In
another word, if the iommu page walk is incoherent, dirty page tracking
doesn't work.

The Intel VT-d specification, Section 3.10 "Snoop Behavior" states:

"Remapping hardware encountering the need to atomically update A/EA/D bits
 in a paging-structure entry that is not snooped will result in a non-
 recoverable fault."

To prevent an IOMMU from being incorrectly configured for dirty page
tracking when it is operating in an incoherent mode, mark SSADS as
supported only when both ecap_slads and ecap_smpwc are supported.

Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
---
 drivers/iommu/intel/iommu.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/intel/iommu.h b/drivers/iommu/intel/iommu.h
index d09b92871659..2c261c069001 100644
--- a/drivers/iommu/intel/iommu.h
+++ b/drivers/iommu/intel/iommu.h
@@ -541,7 +541,8 @@ enum {
 #define pasid_supported(iommu)	(sm_supported(iommu) &&			\
 				 ecap_pasid((iommu)->ecap))
 #define ssads_supported(iommu) (sm_supported(iommu) &&                 \
-				ecap_slads((iommu)->ecap))
+				ecap_slads((iommu)->ecap) &&           \
+				ecap_smpwc(iommu->ecap))
 #define nested_supported(iommu)	(sm_supported(iommu) &&			\
 				 ecap_nest((iommu)->ecap))
 
-- 
2.43.0