iommu/vt-d: Consolidate dmar policy management and force_on logic

Currently the dmar on/off is carried by multiple variables (no_iommu,
dmar_disabled, no_platform_optin, etc.) with error-prone force_on logic
scattered in multiple places.

Unify/centralize the policy/priority management for various force_on
scenarios.

No functional impact except one case - "intel_iommu=off" sets
no_platform_optin which is checked in platform_optin_force_iommu()
but not in detect_intel_iommu(), leading to ACS unnecessarily requested
when iommu could not be forced on later. Now with the unified logic
this becomes more consistent.

Signed-off-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>

drivers/iommu/intel/dmar.c

@@ -915,14 +915,61 @@ dmar_validate_one_drhd(struct acpi_dmar_header *entry, void *arg)
 	return 0;
 }
 
+/*
+ * Centralized helper for deciding the force_on policy
+ *
+ * dmar off policies (for DMA Remapping) are defined from stronger
+ * (more negative values) to weaker (less negative values).
+ *
+ * When a force_on type is passed in, it is associated to a reference
+ * level for comparison. force_on is permitted when dmar is in a
+ * off policy less negative than the reference level (if the policy is
+ * on then the check is always true).
+ *
+ * For supported force_on types:
+ *
+ * - DMAR_FORCEON_TBOOT: tboot strictly requires DMA remapping for secure
+ *   boot hence supersedes any user opts ("iommu=off" or "intel_iommu=off")
+ *   and weaker off policies.
+ *
+ * - DMAR_FORCEON_PLATFORM: external-facing devices requires DMA
+ *   remapping to prevent malicious downstream external devices from
+ *   composing DMA attacks. force_on is permitted only if dmar policy is
+ *   off by build configurations (CONFIG_INTEL_IOMMU_DEFAULT_ON=off).
+ *
+ * In a nutshell, "trusted boot environment" is considered stronger than
+ * "user choices", which in turn is stronger than "platform opt-in hint".
+ */
+bool dmar_can_force_on(enum dmar_force_on force_on)
+{
+	int level;
+
+	switch (force_on) {
+	case DMAR_FORCEON_TBOOT:
+		level = DMAR_USER_OFF;
+		break;
+	case DMAR_FORCEON_PLATFORM:
+		level = DMAR_DEFAULT_OFF;
+		break;
+	default:
+		level = INT_MAX;
+		pr_warn("Unsupported force_on type (%d)\n", force_on);
+		break;
+	}
+
+	return dmar_policy >= level;
+}
+
 static bool dmar_required(void)
 {
-	/* tboot supersedes any user/platform opt */
-	if (!intel_iommu_tboot_noforce && tboot_enabled())
+	if (dmar_policy_on())
 		return true;
 
-	if (!no_iommu && (!dmar_disabled || dmar_platform_optin()))
-		return true;
+	if (!intel_iommu_tboot_noforce && tboot_enabled())
+		return dmar_can_force_on(DMAR_FORCEON_TBOOT);
+
+	if (dmar_platform_optin())
+		return dmar_can_force_on(DMAR_FORCEON_PLATFORM);
 
 	return false;
 }
@@ -936,6 +983,9 @@ void __init detect_intel_iommu(void)
 	};
 
 	down_write(&dmar_global_lock);
+	if (no_iommu)
+		dmar_policy = DMAR_USER_OFF;
+
 	ret = dmar_table_detect();
 	if (!ret)
 		ret = dmar_walk_dmar_table((struct acpi_table_dmar *)dmar_tbl,

drivers/iommu/intel/iommu.c

@@ -199,6 +199,11 @@ static LIST_HEAD(dmar_satc_units);
 
 static void intel_iommu_domain_free(struct iommu_domain *domain);
 
+#ifdef CONFIG_INTEL_IOMMU_DEFAULT_ON
+int dmar_policy = DMAR_ON;
+#else
+int dmar_policy = DMAR_DEFAULT_OFF;
+#endif
 int dmar_disabled = !IS_ENABLED(CONFIG_INTEL_IOMMU_DEFAULT_ON);
 int intel_iommu_sm = IS_ENABLED(CONFIG_INTEL_IOMMU_SCALABLE_MODE_DEFAULT_ON);
 
@@ -240,9 +245,11 @@ static int __init intel_iommu_setup(char *str)
 
 	while (*str) {
 		if (!strncmp(str, "on", 2)) {
+			dmar_policy = DMAR_ON;
 			dmar_disabled = 0;
 			pr_info("IOMMU enabled\n");
 		} else if (!strncmp(str, "off", 3)) {
+			dmar_policy = DMAR_USER_OFF;
 			dmar_disabled = 1;
 			no_platform_optin = 1;
 			pr_info("IOMMU disabled\n");

drivers/iommu/intel/iommu.h

@@ -1351,6 +1351,51 @@ static inline bool ecmd_has_pmu_essential(struct intel_iommu *iommu)
 		DMA_ECMD_ECCAP3_ESSENTIAL;
 }
 
+enum dmar_force_on {
+	DMAR_FORCEON_PLATFORM,
+	DMAR_FORCEON_TBOOT
+};
+
+/*
+ * On policies are positive, with more positive value being stronger.
+ * Off policies are negative, with more negative value being stronger.
+ *
+ * 'dmar' here refers to DMA remapping instead of the dmar/iommu unit.
+ *
+ * - DMAR_FORCE_ON:
+ *     force to turn on (e.g. by tboot or platform opt-in).
+ *
+ * - DMAR_ON:
+ *     turn on by build configuration (CONFIG_INTEL_IOMMU_DEFAULT_ON=on)
+ *     or user opts ("intel_iommu=on").
+ *
+ * - DMAR_DEFAULT_OFF
+ *     turn off by build configuration (CONFIG_INTEL_IOMMU_DEFAULT_ON=off).
+ *
+ * - DMAR_USER_OFF
+ *     turn off by user opts ("intel_iommu=off" or "iommu=off").
+ *
+ * - '0' is invalid, compared to decide the on/off policy
+ *
+ */
+#define DMAR_FORCE_ON		2
+#define DMAR_ON			1
+#define DMAR_DEFAULT_OFF	-1
+#define DMAR_USER_OFF		-2
+extern int dmar_policy;
+
+static inline bool dmar_policy_on(void)
+{
+	return dmar_policy > 0;
+}
+
+static inline bool dmar_policy_off(void)
+{
+	return dmar_policy < 0;
+}
+
+bool dmar_can_force_on(enum dmar_force_on force_on);
+
 extern int dmar_disabled;
 extern int intel_iommu_enabled;
 extern int intel_iommu_tboot_noforce;