iommu/amd: Introduce function to check and enable SNP

To support SNP, IOMMU needs to be enabled, and prohibits IOMMU
configurations where DTE[Mode]=0, which means it cannot be supported with
IOMMU passthrough domain (a.k.a IOMMU_DOMAIN_IDENTITY),
and when AMD IOMMU driver is configured to not use the IOMMU host (v1) page
table. Otherwise, RMP table initialization could cause the system to crash.

The request to enable SNP support in IOMMU must be done before PCI
initialization state of the IOMMU driver because enabling SNP affects
how IOMMU driver sets up IOMMU data structures (i.e. DTE).

Unlike other IOMMU features, SNP feature does not have an enable bit in
the IOMMU control register. Instead, the IOMMU driver introduces
an amd_iommu_snp_en variable to track enabling state of SNP.

Introduce amd_iommu_snp_enable() for other drivers to request enabling
the SNP support in IOMMU, which checks all prerequisites and determines
if the feature can be safely enabled.

Please see the IOMMU spec section 2.12 for further details.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Co-developed-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Link: https://lore.kernel.org/r/20220713225651.20758-7-suravee.suthikulpanit@amd.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/amd/amd_iommu.h

@@ -140,4 +140,6 @@ extern struct dev_table_entry *get_dev_table(struct amd_iommu *iommu);
 
 extern u64 amd_iommu_efr;
 extern u64 amd_iommu_efr2;
+
+extern bool amd_iommu_snp_en;
 #endif

drivers/iommu/amd/init.c

@@ -168,6 +168,10 @@ static int amd_iommu_target_ivhd_type;
 u64 amd_iommu_efr;
 u64 amd_iommu_efr2;
 
+/* SNP is enabled on the system? */
+bool amd_iommu_snp_en;
+EXPORT_SYMBOL(amd_iommu_snp_en);
+
 LIST_HEAD(amd_iommu_pci_seg_list);	/* list of all PCI segments */
 LIST_HEAD(amd_iommu_list);		/* list of all AMD IOMMUs in the
 					   system */
@@ -3556,3 +3560,41 @@ int amd_iommu_pc_set_reg(struct amd_iommu *iommu, u8 bank, u8 cntr, u8 fxn, u64
 
 	return iommu_pc_get_set_reg(iommu, bank, cntr, fxn, value, true);
 }
+
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+int amd_iommu_snp_enable(void)
+{
+	/*
+	 * The SNP support requires that IOMMU must be enabled, and is
+	 * not configured in the passthrough mode.
+	 */
+	if (no_iommu || iommu_default_passthrough()) {
+		pr_err("SNP: IOMMU is disabled or configured in passthrough mode, SNP cannot be supported");
+		return -EINVAL;
+	}
+
+	/*
+	 * Prevent enabling SNP after IOMMU_ENABLED state because this process
+	 * affect how IOMMU driver sets up data structures and configures
+	 * IOMMU hardware.
+	 */
+	if (init_state > IOMMU_ENABLED) {
+		pr_err("SNP: Too late to enable SNP for IOMMU.\n");
+		return -EINVAL;
+	}
+
+	amd_iommu_snp_en = check_feature_on_all_iommus(FEATURE_SNP);
+	if (!amd_iommu_snp_en)
+		return -EINVAL;
+
+	pr_info("SNP enabled\n");
+
+	/* Enforce IOMMU v1 pagetable when SNP is enabled. */
+	if (amd_iommu_pgtable != AMD_IOMMU_V1) {
+		pr_warn("Force to using AMD IOMMU v1 page table due to SNP\n");
+		amd_iommu_pgtable = AMD_IOMMU_V1;
+	}
+
+	return 0;
+}
+#endif

include/linux/amd-iommu.h

@@ -206,4 +206,8 @@ int amd_iommu_pc_get_reg(struct amd_iommu *iommu, u8 bank, u8 cntr, u8 fxn,
 		u64 *value);
 struct amd_iommu *get_amd_iommu(unsigned int idx);
 
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+int amd_iommu_snp_enable(void);
+#endif
+
 #endif /* _ASM_X86_AMD_IOMMU_H */