[PATCH v3 rc] iommu/amd: Do not enable SNP when V2 page table is enabled

[PATCH v3 rc] iommu/amd: Do not enable SNP when V2 page table is enabled

[Vasant Hegde]

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>
---
Changes in v3:
  - Updated description and added Reviewed-by tag.

-Vasant

 drivers/iommu/amd/init.c | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
index 005981c2f96a..0d9b69765099 100644
--- a/drivers/iommu/amd/init.c
+++ b/drivers/iommu/amd/init.c
@@ -3215,28 +3215,29 @@ 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
 }
 
-- 
2.31.1

Re: [PATCH v3 rc] iommu/amd: Do not enable SNP when V2 page table is enabled

[Joerg Roedel]

On Wed, Apr 10, 2024 at 08:57:02AM +0000, Vasant Hegde wrote:
> Changes in v3:
>   - Updated description and added Reviewed-by tag.
> 
> -Vasant
> 
>  drivers/iommu/amd/init.c | 21 +++++++++++----------
>  1 file changed, 11 insertions(+), 10 deletions(-)

Applied, thanks.