iommu/amd: Do not enable SNP when V2 page table is enabled

DTE[Mode]=0 is not supported when SNP is enabled in the host. That means
to support SNP, IOMMU must be configured with V1 page table (See IOMMU
spec [1] for the details). If user passes kernel command line to configure
IOMMU domains with v2 page table (amd_iommu=pgtbl_v2) then disable SNP
as the user asked by not forcing the page table to v1.

[1] https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/specifications/48882_IOMMU.pdf

Cc: Ashish Kalra <ashish.kalra@amd.com>
Cc: Michael Roth <michael.roth@amd.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20240410085702.31869-1-vasant.hegde@amd.comSigned-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/amd/init.c

@@ -3232,28 +3232,29 @@ static void iommu_snp_enable(void)
 		return;
 	/*
 	 * The SNP support requires that IOMMU must be enabled, and is
-	 * not configured in the passthrough mode.
+	 * configured with V1 page table (DTE[Mode] = 0 is not supported).
 	 */
 	if (no_iommu || iommu_default_passthrough()) {
 		pr_err("SNP: IOMMU disabled or configured in passthrough mode, SNP cannot be supported.\n");
-		cc_platform_clear(CC_ATTR_HOST_SEV_SNP);
-		return;
+		goto disable_snp;
+	}
+
+	if (amd_iommu_pgtable != AMD_IOMMU_V1) {
+		pr_warn("SNP: IOMMU is configured with V2 page table mode, SNP cannot be supported.\n");
+		goto disable_snp;
 	}
 
 	amd_iommu_snp_en = check_feature(FEATURE_SNP);
 	if (!amd_iommu_snp_en) {
 		pr_err("SNP: IOMMU SNP feature not enabled, SNP cannot be supported.\n");
-		cc_platform_clear(CC_ATTR_HOST_SEV_SNP);
-		return;
+		goto disable_snp;
 	}
 
 	pr_info("IOMMU SNP support enabled.\n");
+	return;
 
-	/* Enforce IOMMU v1 pagetable when SNP is enabled. */
-	if (amd_iommu_pgtable != AMD_IOMMU_V1) {
-		pr_warn("Forcing use of AMD IOMMU v1 page table due to SNP.\n");
-		amd_iommu_pgtable = AMD_IOMMU_V1;
-	}
+disable_snp:
+	cc_platform_clear(CC_ATTR_HOST_SEV_SNP);
 #endif
 }