KVM: arm64: Always return generic v8 as the preferred target

Userspace selecting an implementation-specific vCPU target has been
completely useless for a very long time. Let's go whole hog and start
returning the generic v8 target across all implementations as the
preferred target.

Uphold the pre-existing behavior by tolerating either the generic target
or an implementation-specific target if the vCPU happens to be running
on one of the lucky few parts.

Acked-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230710193140.1706399-5-oliver.upton@linux.dev

Author: oupton

arch/arm64/include/asm/kvm_host.h

@@ -898,7 +898,6 @@ struct kvm_vcpu_stat {
 	u64 exits;
 };
 
-void 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);
 int kvm_arm_get_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg);

arch/arm64/kvm/arm.c

@@ -1250,7 +1250,8 @@ static int kvm_vcpu_set_target(struct kvm_vcpu *vcpu,
 {
 	int ret;
 
-	if (init->target != kvm_target_cpu())
+	if (init->target != KVM_ARM_TARGET_GENERIC_V8 &&
+	    init->target != kvm_target_cpu())
 		return -EINVAL;
 
 	ret = kvm_vcpu_init_check_features(vcpu, init);
@@ -1585,9 +1586,9 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
 		return kvm_vm_ioctl_set_device_addr(kvm, &dev_addr);
 	}
 	case KVM_ARM_PREFERRED_TARGET: {
-		struct kvm_vcpu_init init;
-
-		kvm_vcpu_preferred_target(&init);
+		struct kvm_vcpu_init init = {
+			.target = KVM_ARM_TARGET_GENERIC_V8,
+		};
 
 		if (copy_to_user(argp, &init, sizeof(init)))
 			return -EFAULT;

arch/arm64/kvm/guest.c

@@ -884,21 +884,6 @@ u32 __attribute_const__ kvm_target_cpu(void)
 	return KVM_ARM_TARGET_GENERIC_V8;
 }
 
-void kvm_vcpu_preferred_target(struct kvm_vcpu_init *init)
-{
-	u32 target = kvm_target_cpu();
-
-	memset(init, 0, sizeof(*init));
-
-	/*
-	 * For now, we don't return any features.
-	 * In future, we might use features to return target
-	 * specific features available for the preferred
-	 * target type.
-	 */
-	init->target = (__u32)target;
-}
-
 int kvm_arch_vcpu_ioctl_get_fpu(struct kvm_vcpu *vcpu, struct kvm_fpu *fpu)
 {
 	return -EINVAL;