arm/arm64: KVM: Clarify KVM_ARM_VCPU_INIT ABI

It is not clear that this ioctl can be called multiple times for a given
vcpu. Userspace already does this, so clarify the ABI.

Also specify that userspace is expected to always make secondary and
subsequent calls to the ioctl with the same parameters for the VCPU as
the initial call (which userspace also already does).

Add code to check that userspace doesn't violate that ABI in the future,
and move the kvm_vcpu_set_target() function which is currently
duplicated between the 32-bit and 64-bit versions in guest.c to a common
static function in arm.c, shared between both architectures.

Acked-by: Marc Zyngier <[e-mail]>
Signed-off-by: Christoffer Dall <[e-mail]>

Documentation/virtual/kvm/api.txt

···
 Note that because some registers reflect machine topology, all vcpus
 should be created before this ioctl is invoked.
 
+Userspace can call this function multiple times for a given vcpu, including
+after the vcpu has been run. This will reset the vcpu to its initial
+state. All calls to this function after the initial call must use the same
+target and same set of feature flags, otherwise EINVAL will be returned.
+
 Possible features:
 	- KVM_ARM_VCPU_POWER_OFF: Starts the CPU in a power-off state.
 	  Depends on KVM_CAP_ARM_PSCI.  If not set, the CPU will be powered on

arch/arm/include/asm/kvm_host.h

···
 	u32 halt_wakeup;
 };
 
-int kvm_vcpu_set_target(struct kvm_vcpu *vcpu,
-			const struct kvm_vcpu_init *init);
 int kvm_vcpu_preferred_target(struct kvm_vcpu_init *init);
 unsigned long kvm_arm_num_regs(struct kvm_vcpu *vcpu);
 int kvm_arm_copy_reg_indices(struct kvm_vcpu *vcpu, u64 __user *indices);

arch/arm/kvm/arm.c

···
 {
 	/* Force users to call KVM_ARM_VCPU_INIT */
 	vcpu->arch.target = -1;
+	bitmap_zero(vcpu->arch.features, KVM_VCPU_MAX_FEATURES);
 
 	/* Set up the timer */
 	kvm_timer_vcpu_init(vcpu);
···
 
 	return -EINVAL;
 }
+
+static int kvm_vcpu_set_target(struct kvm_vcpu *vcpu,
+			       const struct kvm_vcpu_init *init)
+{
+	unsigned int i;
+	int phys_target = kvm_target_cpu();
+
+	if (init->target != phys_target)
+		return -EINVAL;
+
+	/*
+	 * Secondary and subsequent calls to KVM_ARM_VCPU_INIT must
+	 * use the same target.
+	 */
+	if (vcpu->arch.target != -1 && vcpu->arch.target != init->target)
+		return -EINVAL;
+
+	/* -ENOENT for unknown features, -EINVAL for invalid combinations. */
+	for (i = 0; i < sizeof(init->features) * 8; i++) {
+		bool set = (init->features[i / 32] & (1 << (i % 32)));
+
+		if (set && i >= KVM_VCPU_MAX_FEATURES)
+			return -ENOENT;
+
+		/*
+		 * Secondary and subsequent calls to KVM_ARM_VCPU_INIT must
+		 * use the same feature set.
+		 */
+		if (vcpu->arch.target != -1 && i < KVM_VCPU_MAX_FEATURES &&
+		    test_bit(i, vcpu->arch.features) != set)
+			return -EINVAL;
+
+		if (set)
+			set_bit(i, vcpu->arch.features);
+	}
+
+	vcpu->arch.target = phys_target;
+
+	/* Now we know what it is, we can reset it. */
+	return kvm_reset_vcpu(vcpu);
+}
+
 
 static int kvm_arch_vcpu_ioctl_vcpu_init(struct kvm_vcpu *vcpu,
 					 struct kvm_vcpu_init *init)

arch/arm/kvm/guest.c

···
 	}
 }
 
-int kvm_vcpu_set_target(struct kvm_vcpu *vcpu,
-			const struct kvm_vcpu_init *init)
-{
-	unsigned int i;
-
-	/* We can only cope with guest==host and only on A15/A7 (for now). */
-	if (init->target != kvm_target_cpu())
-		return -EINVAL;
-
-	vcpu->arch.target = init->target;
-	bitmap_zero(vcpu->arch.features, KVM_VCPU_MAX_FEATURES);
-
-	/* -ENOENT for unknown features, -EINVAL for invalid combinations. */
-	for (i = 0; i < sizeof(init->features) * 8; i++) {
-		if (test_bit(i, (void *)init->features)) {
-			if (i >= KVM_VCPU_MAX_FEATURES)
-				return -ENOENT;
-			set_bit(i, vcpu->arch.features);
-		}
-	}
-
-	/* Now we know what it is, we can reset it. */
-	return kvm_reset_vcpu(vcpu);
-}
-
 int kvm_vcpu_preferred_target(struct kvm_vcpu_init *init)
 {
 	int target = kvm_target_cpu();

arch/arm64/include/asm/kvm_host.h

···
 	u32 halt_wakeup;
 };
 
-int kvm_vcpu_set_target(struct kvm_vcpu *vcpu,
-			const struct kvm_vcpu_init *init);
 int kvm_vcpu_preferred_target(struct kvm_vcpu_init *init);
 unsigned long kvm_arm_num_regs(struct kvm_vcpu *vcpu);
 int kvm_arm_copy_reg_indices(struct kvm_vcpu *vcpu, u64 __user *indices);

arch/arm64/kvm/guest.c

···
 	return -EINVAL;
 }
 
-int kvm_vcpu_set_target(struct kvm_vcpu *vcpu,
-			const struct kvm_vcpu_init *init)
-{
-	unsigned int i;
-	int phys_target = kvm_target_cpu();
-
-	if (init->target != phys_target)
-		return -EINVAL;
-
-	vcpu->arch.target = phys_target;
-	bitmap_zero(vcpu->arch.features, KVM_VCPU_MAX_FEATURES);
-
-	/* -ENOENT for unknown features, -EINVAL for invalid combinations. */
-	for (i = 0; i < sizeof(init->features) * 8; i++) {
-		if (init->features[i / 32] & (1 << (i % 32))) {
-			if (i >= KVM_VCPU_MAX_FEATURES)
-				return -ENOENT;
-			set_bit(i, vcpu->arch.features);
-		}
-	}
-
-	/* Now we know what it is, we can reset it. */
-	return kvm_reset_vcpu(vcpu);
-}
-
 int kvm_vcpu_preferred_target(struct kvm_vcpu_init *init)
 {
 	int target = kvm_target_cpu();