[PATCH 3/8] iommu/vt-d: Require CMPXCHG16B for PASID support

[Lu Baolu]

The Intel IOMMU driver is moving toward using the generic entry_sync
library for PASID table entry updates. This library requires 128-bit
atomic write operations (cmpxchg128) to update 512-bit PASID entries in
atomic quanta, ensuring the hardware never observes a torn entry.

On x86_64, 128-bit atomicity is provided by the CMPXCHG16B instruction.
Update the driver to:

1. Limit INTEL_IOMMU to X86_64, as 128-bit atomic operations are not
   available on 32-bit x86.
2. Gate pasid_supported() on the presence of X86_FEATURE_CX16.
3. Provide a boot-time warning if a PASID-capable IOMMU is detected on
   a CPU lacking the required instruction.

Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
---
 drivers/iommu/intel/Kconfig | 2 +-
 drivers/iommu/intel/iommu.h | 3 ++-
 drivers/iommu/intel/iommu.c | 4 ++++
 3 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/iommu/intel/Kconfig b/drivers/iommu/intel/Kconfig
index 7fa31b9d4ef4..fee7fea9dfcb 100644
--- a/drivers/iommu/intel/Kconfig
+++ b/drivers/iommu/intel/Kconfig
@@ -11,7 +11,7 @@ config DMAR_DEBUG
 
 config INTEL_IOMMU
 	bool "Support for Intel IOMMU using DMA Remapping Devices"
-	depends on PCI_MSI && ACPI && X86
+	depends on PCI_MSI && ACPI && X86_64
 	select IOMMU_API
 	select GENERIC_PT
 	select IOMMU_PT
diff --git a/drivers/iommu/intel/iommu.h b/drivers/iommu/intel/iommu.h
index 599913fb65d5..54b58d01d0cb 100644
--- a/drivers/iommu/intel/iommu.h
+++ b/drivers/iommu/intel/iommu.h
@@ -535,7 +535,8 @@ enum {
 
 #define sm_supported(iommu)	(intel_iommu_sm && ecap_smts((iommu)->ecap))
 #define pasid_supported(iommu)	(sm_supported(iommu) &&			\
-				 ecap_pasid((iommu)->ecap))
+				 ecap_pasid((iommu)->ecap) &&		\
+				 boot_cpu_has(X86_FEATURE_CX16))
 #define ssads_supported(iommu) (sm_supported(iommu) &&                 \
 				ecap_slads((iommu)->ecap) &&           \
 				ecap_smpwc(iommu->ecap))
diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
index ef7613b177b9..5369526e89d0 100644
--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -2647,6 +2647,10 @@ int __init intel_iommu_init(void)
 			pr_info_once("IOMMU batching disallowed due to virtualization\n");
 			iommu_set_dma_strict();
 		}
+
+		if (ecap_pasid(iommu->ecap) && !boot_cpu_has(X86_FEATURE_CX16))
+			pr_info_once("PASID disabled due to lack of CMPXCHG16B support.\n");
+
 		iommu_device_sysfs_add(&iommu->iommu, NULL,
 				       intel_iommu_groups,
 				       "%s", iommu->name);
-- 
2.43.0