linux-kernel - [PATCH 1/1] KVM: Add KVM_GET_REG

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Message-ID: <20260125054322.1237687-1-liushuyu@aosc.io>
Date: Sun, 25 Jan 2026 13:43:22 +0800
From: Zixing Liu <liushuyu@...c.io>
To: WANG Xuerui <kernel@...0n.name>,
	Huacai Chen <chenhuacai@...nel.org>,
	Bibo Mao <maobibo@...ngson.cn>
Cc: Kexy Biscuit <kexybiscuit@...c.io>,
	Mingcong Bai <jeffbai@...c.io>,
	Zixing Liu <liushuyu@...c.io>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Jonathan Corbet <corbet@....net>,
	Tianrui Zhao <zhaotianrui@...ngson.cn>,
	Paul Walmsley <pjw@...nel.org>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Alexandre Ghiti <alex@...ti.fr>,
	Miguel Ojeda <ojeda@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>,
	kvm@...r.kernel.org,
	linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	loongarch@...ts.linux.dev,
	linux-riscv@...ts.infradead.org,
	rust-for-linux@...r.kernel.org
Subject: [PATCH 1/1] KVM: Add KVM_GET_REG_LIST ioctl for LoongArch

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

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@...c.io>
---

For example, this ioctl could be used by rust-vmm/rust-kvm or maybe
VirtualBox-kvm in the future.

 Documentation/virt/kvm/api.rst |  2 +-
 arch/loongarch/kvm/vcpu.c      | 69 ++++++++++++++++++++++++++++++++++
 2 files changed, 70 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..b884eb9c76aa 100644
--- a/arch/loongarch/kvm/vcpu.c
+++ b/arch/loongarch/kvm/vcpu.c
@@ -1186,6 +1186,57 @@ static int kvm_loongarch_vcpu_set_attr(struct kvm_vcpu *vcpu,
 	return ret;
 }
 
+static unsigned long kvm_loongarch_num_lbt_regs(void)
+{
+	/* +1 for the LBT_FTOP flag (inside arch.fpu) */
+	return sizeof(struct loongarch_lbt) / sizeof(unsigned long) + 1;
+}
+
+static unsigned long kvm_loongarch_num_regs(struct kvm_vcpu *vcpu)
+{
+	/* +1 for the KVM_REG_LOONGARCH_COUNTER register */
+	unsigned long res = CSR_MAX_NUMS + KVM_MAX_CPUCFG_REGS + 1;
+
+	if (kvm_guest_has_lbt(&vcpu->arch))
+		res += kvm_loongarch_num_lbt_regs();
+
+	return res;
+}
+
+static int kvm_loongarch_copy_reg_indices(struct kvm_vcpu *vcpu,
+					  u64 __user *uindices)
+{
+	u64 reg;
+	unsigned int i;
+
+	for (i = 0; i < CSR_MAX_NUMS; i++) {
+		reg = KVM_IOC_CSRID(i);
+		if (put_user(reg, uindices++))
+			return -EFAULT;
+	}
+
+	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 <= kvm_loongarch_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 +1302,24 @@ 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