KVM: arm64: Factor out core register ID enumeration

In preparation for adding logic to filter out some KVM_REG_ARM_CORE
registers from the KVM_GET_REG_LIST output, this patch factors out
the core register enumeration into a separate function and rebuilds
num_core_regs() on top of it.

This may be a little more expensive (depending on how good a job
the compiler does of specialising the code), but KVM_GET_REG_LIST
is not a hot path.

This will make it easier to consolidate ID filtering code in one
place.

No functional change.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Julien Thierry <julien.thierry@arm.com>
Tested-by: zhang.lei <zhang.lei@jp.fujitsu.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

arch/arm64/kvm/guest.c

@@ -23,6 +23,7 @@
 #include <linux/err.h>
 #include <linux/kvm_host.h>
 #include <linux/module.h>
+#include <linux/stddef.h>
 #include <linux/string.h>
 #include <linux/vmalloc.h>
 #include <linux/fs.h>
@@ -194,9 +195,28 @@ int kvm_arch_vcpu_ioctl_set_regs(struct kvm_vcpu *vcpu, struct kvm_regs *regs)
 	return -EINVAL;
 }
 
+static int kvm_arm_copy_core_reg_indices(u64 __user *uindices)
+{
+	unsigned int i;
+	int n = 0;
+	const u64 core_reg = KVM_REG_ARM64 | KVM_REG_SIZE_U64 | KVM_REG_ARM_CORE;
+
+	for (i = 0; i < sizeof(struct kvm_regs) / sizeof(__u32); i++) {
+		if (uindices) {
+			if (put_user(core_reg | i, uindices))
+				return -EFAULT;
+			uindices++;
+		}
+
+		n++;
+	}
+
+	return n;
+}
+
 static unsigned long num_core_regs(void)
 {
-	return sizeof(struct kvm_regs) / sizeof(__u32);
+	return kvm_arm_copy_core_reg_indices(NULL);
 }
 
 /**
@@ -276,15 +296,12 @@ unsigned long kvm_arm_num_regs(struct kvm_vcpu *vcpu)
  */
 int kvm_arm_copy_reg_indices(struct kvm_vcpu *vcpu, u64 __user *uindices)
 {
-	unsigned int i;
-	const u64 core_reg = KVM_REG_ARM64 | KVM_REG_SIZE_U64 | KVM_REG_ARM_CORE;
 	int ret;
 
-	for (i = 0; i < sizeof(struct kvm_regs) / sizeof(__u32); i++) {
-		if (put_user(core_reg | i, uindices))
-			return -EFAULT;
-		uindices++;
-	}
+	ret = kvm_arm_copy_core_reg_indices(uindices);
+	if (ret)
+		return ret;
+	uindices += ret;
 
 	ret = kvm_arm_copy_fw_reg_indices(vcpu, uindices);
 	if (ret)