KVM: arm64: Add helper for last ditch idreg adjustments

We already have to perform a set of last-chance adjustments for
NV purposes. We will soon have to do the same for the GIC, so
introduce a helper for that exact purpose.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240827152517.3909653-5-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/kvm/arm.c

@@ -46,6 +46,8 @@
 #include <kvm/arm_pmu.h>
 #include <kvm/arm_psci.h>
 
+#include "sys_regs.h"
+
 static enum kvm_mode kvm_mode = KVM_MODE_DEFAULT;
 
 enum kvm_wfx_trap_policy {
@@ -821,15 +823,13 @@ int kvm_arch_vcpu_run_pid_change(struct kvm_vcpu *vcpu)
 			return ret;
 	}
 
-	if (vcpu_has_nv(vcpu)) {
-		ret = kvm_init_nv_sysregs(vcpu->kvm);
-		if (ret)
-			return ret;
-	}
+	ret = kvm_finalize_sys_regs(vcpu);
+	if (ret)
+		return ret;
 
 	/*
-	 * This needs to happen after NV has imposed its own restrictions on
-	 * the feature set
+	 * This needs to happen after any restriction has been applied
+	 * to the feature set.
 	 */
 	kvm_calculate_traps(vcpu);
 

arch/arm64/kvm/nested.c

@@ -954,19 +954,16 @@ static void set_sysreg_masks(struct kvm *kvm, int sr, u64 res0, u64 res1)
 int kvm_init_nv_sysregs(struct kvm *kvm)
 {
 	u64 res0, res1;
-	int ret = 0;
 
-	mutex_lock(&kvm->arch.config_lock);
+	lockdep_assert_held(&kvm->arch.config_lock);
 
 	if (kvm->arch.sysreg_masks)
-		goto out;
+		return 0;
 
 	kvm->arch.sysreg_masks = kzalloc(sizeof(*(kvm->arch.sysreg_masks)),
 					 GFP_KERNEL_ACCOUNT);
-	if (!kvm->arch.sysreg_masks) {
-		ret = -ENOMEM;
-		goto out;
-	}
+	if (!kvm->arch.sysreg_masks)
+		return -ENOMEM;
 
 	limit_nv_id_regs(kvm);
 
@@ -1195,8 +1192,6 @@ int kvm_init_nv_sysregs(struct kvm *kvm)
 	if (!kvm_has_feat(kvm, ID_AA64PFR0_EL1, AMU, V1P1))
 		res0 |= ~(res0 | res1);
 	set_sysreg_masks(kvm, HAFGRTR_EL2, res0, res1);
-out:
-	mutex_unlock(&kvm->arch.config_lock);
 
-	return ret;
+	return 0;
 }

arch/arm64/kvm/sys_regs.c

@@ -4620,6 +4620,29 @@ out:
 	mutex_unlock(&kvm->arch.config_lock);
 }
 
+/*
+ * Perform last adjustments to the ID registers that are implied by the
+ * configuration outside of the ID regs themselves, as well as any
+ * initialisation that directly depend on these ID registers (such as
+ * RES0/RES1 behaviours). This is not the place to configure traps though.
+ *
+ * Because this can be called once per CPU, changes must be idempotent.
+ */
+int kvm_finalize_sys_regs(struct kvm_vcpu *vcpu)
+{
+	struct kvm *kvm = vcpu->kvm;
+
+	guard(mutex)(&kvm->arch.config_lock);
+
+	if (vcpu_has_nv(vcpu)) {
+		int ret = kvm_init_nv_sysregs(kvm);
+		if (ret)
+			return ret;
+	}
+
+	return 0;
+}
+
 int __init kvm_sys_reg_table_init(void)
 {
 	bool valid = true;

arch/arm64/kvm/sys_regs.h

@@ -235,6 +235,8 @@ int kvm_sys_reg_set_user(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg,
 
 bool triage_sysreg_trap(struct kvm_vcpu *vcpu, int *sr_index);
 
+int kvm_finalize_sys_regs(struct kvm_vcpu *vcpu);
+
 #define AA32(_x)	.aarch32_map = AA32_##_x
 #define Op0(_x) 	.Op0 = _x
 #define Op1(_x) 	.Op1 = _x