According to the AMD IOMMU spec, IOMMU hardware reads the entire DTE
in a single 256-bit transaction. It is recommended to update DTE using
128-bit operation followed by an INVALIDATE_DEVTAB_ENTYRY command when
the IV=1b or V=1b before the change.

According to the AMD BIOS and Kernel Developer's Guide (BDKG) dated back
to family 10h Processor [1], which is the first introduction of AMD IOMMU,
AMD processor always has CPUID Fn0000_0001_ECX[CMPXCHG16B]=1.
Therefore, it is safe to assume cmpxchg128 is available with all AMD
processor w/ IOMMU.

In addition, the CMPXCHG16B feature has already been checked separately
before enabling the GA, XT, and GAM modes. Consolidate the detection logic,
and fail the IOMMU initialization if the feature is not supported.

[1] https://www.amd.com/content/dam/amd/en/documents/archived-tech-docs/programmer-references/31116.pdf

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
---
 drivers/iommu/amd/init.c | 23 +++++++++--------------
 1 file changed, 9 insertions(+), 14 deletions(-)

diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
index 3a7b2b0472fa..c1607b29ebf4 100644
--- a/drivers/iommu/amd/init.c
+++ b/drivers/iommu/amd/init.c
@@ -1752,13 +1752,8 @@ static int __init init_iommu_one(struct amd_iommu *iommu, struct ivhd_header *h,
 		else
 			iommu->mmio_phys_end = MMIO_CNTR_CONF_OFFSET;
 
-		/*
-		 * Note: GA (128-bit IRTE) mode requires cmpxchg16b supports.
-		 * GAM also requires GA mode. Therefore, we need to
-		 * check cmpxchg16b support before enabling it.
-		 */
-		if (!boot_cpu_has(X86_FEATURE_CX16) ||
-		    ((h->efr_attr & (0x1 << IOMMU_FEAT_GASUP_SHIFT)) == 0))
+		/* GAM requires GA mode. */
+		if ((h->efr_attr & (0x1 << IOMMU_FEAT_GASUP_SHIFT)) == 0)
 			amd_iommu_guest_ir = AMD_IOMMU_GUEST_IR_LEGACY;
 		break;
 	case 0x11:
@@ -1768,13 +1763,8 @@ static int __init init_iommu_one(struct amd_iommu *iommu, struct ivhd_header *h,
 		else
 			iommu->mmio_phys_end = MMIO_CNTR_CONF_OFFSET;
 
-		/*
-		 * Note: GA (128-bit IRTE) mode requires cmpxchg16b supports.
-		 * XT, GAM also requires GA mode. Therefore, we need to
-		 * check cmpxchg16b support before enabling them.
-		 */
-		if (!boot_cpu_has(X86_FEATURE_CX16) ||
-		    ((h->efr_reg & (0x1 << IOMMU_EFR_GASUP_SHIFT)) == 0)) {
+		/* XT and GAM require GA mode. */
+		if ((h->efr_reg & (0x1 << IOMMU_EFR_GASUP_SHIFT)) == 0) {
 			amd_iommu_guest_ir = AMD_IOMMU_GUEST_IR_LEGACY;
 			break;
 		}
@@ -3028,6 +3018,11 @@ static int __init early_amd_iommu_init(void)
 		return -EINVAL;
 	}
 
+	if (!boot_cpu_has(X86_FEATURE_CX16)) {
+		pr_err("Failed to initialize. The CMPXCHG16B feature is required.\n");
+		return -EINVAL;
+	}
+
 	/*
 	 * Validate checksum here so we don't need to do it when
 	 * we actually parse the table
-- 
2.34.1