This ioctl can be used by the userspace applications to determine which
(special) registers are get/set-able in a meaningful way.

This can be very useful for cross-platform VMMs so that they do not have
to hardcode register indices for each supported architectures.

Signed-off-by: Zixing Liu <liushuyu@aosc.io>
---
 Documentation/virt/kvm/api.rst |  2 +-
 arch/loongarch/kvm/vcpu.c      | 81 ++++++++++++++++++++++++++++++++++
 2 files changed, 82 insertions(+), 1 deletion(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 01a3abef8abb..f46dd8be282f 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -3603,7 +3603,7 @@ VCPU matching underlying host.
 ---------------------
 
 :Capability: basic
-:Architectures: arm64, mips, riscv, x86 (if KVM_CAP_ONE_REG)
+:Architectures: arm64, loongarch, mips, riscv, x86 (if KVM_CAP_ONE_REG)
 :Type: vcpu ioctl
 :Parameters: struct kvm_reg_list (in/out)
 :Returns: 0 on success; -1 on error
diff --git a/arch/loongarch/kvm/vcpu.c b/arch/loongarch/kvm/vcpu.c
index 656b954c1134..fb8001deadc9 100644
--- a/arch/loongarch/kvm/vcpu.c
+++ b/arch/loongarch/kvm/vcpu.c
@@ -14,6 +14,8 @@
 #define CREATE_TRACE_POINTS
 #include "trace.h"
 
+#define NUM_LBT_REGS 6
+
 const struct _kvm_stats_desc kvm_vcpu_stats_desc[] = {
 	KVM_GENERIC_VCPU_STATS(),
 	STATS_DESC_COUNTER(VCPU, int_exits),
@@ -1186,6 +1188,67 @@ static int kvm_loongarch_vcpu_set_attr(struct kvm_vcpu *vcpu,
 	return ret;
 }
 
+static int kvm_loongarch_walk_csrs(struct kvm_vcpu *vcpu, u64 __user *uindices)
+{
+	unsigned int i, count;
+
+	for (i = 0, count = 0; i < CSR_MAX_NUMS; i++) {
+		if (!(get_gcsr_flag(i) & (SW_GCSR | HW_GCSR)))
+			continue;
+		const u64 reg = KVM_IOC_CSRID(i);
+		if (uindices && put_user(reg, uindices++))
+			return -EFAULT;
+		count++;
+	}
+
+	return count;
+}
+
+static unsigned long kvm_loongarch_num_regs(struct kvm_vcpu *vcpu)
+{
+	/* +1 for the KVM_REG_LOONGARCH_COUNTER register */
+	unsigned long res =
+		kvm_loongarch_walk_csrs(vcpu, NULL) + KVM_MAX_CPUCFG_REGS + 1;
+
+	if (kvm_guest_has_lbt(&vcpu->arch))
+		res += NUM_LBT_REGS;
+
+	return res;
+}
+
+static int kvm_loongarch_copy_reg_indices(struct kvm_vcpu *vcpu,
+					  u64 __user *uindices)
+{
+	u64 reg;
+	unsigned int i;
+
+	i = kvm_loongarch_walk_csrs(vcpu, uindices);
+	if (i < 0)
+		return i;
+	uindices += i;
+
+	for (i = 0; i < KVM_MAX_CPUCFG_REGS; i++) {
+		reg = KVM_IOC_CPUCFG(i);
+		if (put_user(reg, uindices++))
+			return -EFAULT;
+	}
+
+	reg = KVM_REG_LOONGARCH_COUNTER;
+	if (put_user(reg, uindices++))
+		return -EFAULT;
+
+	if (!kvm_guest_has_lbt(&vcpu->arch))
+		return 0;
+
+	for (i = 1; i <= NUM_LBT_REGS; i++) {
+		reg = (KVM_REG_LOONGARCH_LBT | KVM_REG_SIZE_U64 | i);
+		if (put_user(reg, uindices++))
+			return -EFAULT;
+	}
+
+	return 0;
+}
+
 long kvm_arch_vcpu_ioctl(struct file *filp,
 			 unsigned int ioctl, unsigned long arg)
 {
@@ -1251,6 +1314,24 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
 		r = kvm_loongarch_vcpu_set_attr(vcpu, &attr);
 		break;
 	}
+	case KVM_GET_REG_LIST: {
+		struct kvm_reg_list __user *user_list = argp;
+		struct kvm_reg_list reg_list;
+		unsigned n;
+
+		r = -EFAULT;
+		if (copy_from_user(&reg_list, user_list, sizeof(reg_list)))
+			break;
+		n = reg_list.n;
+		reg_list.n = kvm_loongarch_num_regs(vcpu);
+		if (copy_to_user(user_list, &reg_list, sizeof(reg_list)))
+			break;
+		r = -E2BIG;
+		if (n < reg_list.n)
+			break;
+		r = kvm_loongarch_copy_reg_indices(vcpu, user_list->reg);
+		break;
+	}
 	default:
 		r = -ENOIOCTLCMD;
 		break;
-- 
2.52.0