[PATCH 1/1] iommu/vt-d: Disallow SVA if page walk is not coherent

[PATCH 1/1] iommu/vt-d: Disallow SVA if page walk is not coherent

[Lu Baolu]

Hardware implementations report Scalable-Mode Page-walk Coherency Support
via the SMPWCS field in the extended capability register. If the hardware
does not support page-walk coherency, a clflush is required every time
the page table entries (which are walked by the IOMMU hardware) are
updated.

In the SVA case, page tables are managed by the CPU mm core, not by the
IOMMU driver. Because the IOMMU driver has no way of knowing whether the
CPU page table management code has ensured coherency via clflush, the
driver must deny SVA if the hardware does not support coherent paging.

Fixes: ff3dc6521f78 ("iommu/vt-d: Fix CPU and IOMMU SVM feature matching checks")
Cc: stable@vger.kernel.org
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
---
 drivers/iommu/intel/svm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/iommu/intel/svm.c b/drivers/iommu/intel/svm.c
index 0636987f03c8..432e35723ce9 100644
--- a/drivers/iommu/intel/svm.c
+++ b/drivers/iommu/intel/svm.c
@@ -27,7 +27,7 @@
 
 void intel_svm_check(struct intel_iommu *iommu)
 {
-	if (!pasid_supported(iommu))
+	if (!pasid_supported(iommu) || !ecap_smpwc(iommu->ecap))
 		return;
 
 	if (cpu_feature_enabled(X86_FEATURE_GBPAGES) &&
-- 
2.43.0

Re: [PATCH 1/1] iommu/vt-d: Disallow SVA if page walk is not coherent

[Jason Gunthorpe]

On Thu, Jul 16, 2026 at 01:35:53PM +0800, Lu Baolu wrote:
> Hardware implementations report Scalable-Mode Page-walk Coherency Support
> via the SMPWCS field in the extended capability register. If the hardware
> does not support page-walk coherency, a clflush is required every time
> the page table entries (which are walked by the IOMMU hardware) are
> updated.
> 
> In the SVA case, page tables are managed by the CPU mm core, not by the
> IOMMU driver. Because the IOMMU driver has no way of knowing whether the
> CPU page table management code has ensured coherency via clflush, the
> driver must deny SVA if the hardware does not support coherent paging.
> 
> Fixes: ff3dc6521f78 ("iommu/vt-d: Fix CPU and IOMMU SVM feature matching checks")
> Cc: stable@vger.kernel.org
> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
> ---
>  drivers/iommu/intel/svm.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Jason

Re: [PATCH 1/1] iommu/vt-d: Disallow SVA if page walk is not coherent

[Samiullah Khawaja]

On Thu, Jul 16, 2026 at 01:35:53PM +0800, Lu Baolu wrote:
>Hardware implementations report Scalable-Mode Page-walk Coherency Support
>via the SMPWCS field in the extended capability register. If the hardware
>does not support page-walk coherency, a clflush is required every time
>the page table entries (which are walked by the IOMMU hardware) are
>updated.
>
>In the SVA case, page tables are managed by the CPU mm core, not by the
>IOMMU driver. Because the IOMMU driver has no way of knowing whether the
>CPU page table management code has ensured coherency via clflush, the
>driver must deny SVA if the hardware does not support coherent paging.
>
>Fixes: ff3dc6521f78 ("iommu/vt-d: Fix CPU and IOMMU SVM feature matching checks")
>Cc: stable@vger.kernel.org
>Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
>---
> drivers/iommu/intel/svm.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/drivers/iommu/intel/svm.c b/drivers/iommu/intel/svm.c
>index 0636987f03c8..432e35723ce9 100644
>--- a/drivers/iommu/intel/svm.c
>+++ b/drivers/iommu/intel/svm.c
>@@ -27,7 +27,7 @@
>
> void intel_svm_check(struct intel_iommu *iommu)
> {
>-	if (!pasid_supported(iommu))
>+	if (!pasid_supported(iommu) || !ecap_smpwc(iommu->ecap))
> 		return;
>
> 	if (cpu_feature_enabled(X86_FEATURE_GBPAGES) &&
>-- 
>2.43.0
>
>

Reviewed-by: Samiullah Khawaja <skhawaja@google.com>

Thanks,
Sami