Re: [PATCH 1/1] KVM: Add KVM_GET_REG_LIST ioctl for LoongArch
From: Bibo Mao
Date: Sun Jan 25 2026 - 23:00:58 EST
On 2026/1/26 上午11:38, liushuyu wrote:
Hi Bibo,It will be better if CSR register list can be categorized, for example with LoongArch CPU manual CSR register is split into 8 types from chapter 7.4 -- 7.11
Did you mean we should only return the CSR registers initialized in this
On 2026/1/25 下午1:43, Zixing Liu wrote:
This ioctl can be used by userspace applications to determine whichCSR_MAX_NUMS is max number of accessible CSR registers, instead only
(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@xxxxxxx>
---
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;
+ }
part of them is used by vCPU model. By my understanding, there will be
no much meaning if CSR_MAX_NUMS is returned. And I think it will be
better if real CSR register id and number is returned.
function
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/loongarch/kvm/main.c?id=63804fed149a6750ffd28610c5c1c98cce6bd377#n48
?
That looks like a very large list and the values of the register IDs are
not fully continuous. If that is the case, how do we maintain the
get/set-able CSR register list?
At the beginning, there is big array with size CSR_MAX_NUMS without any category, it can be fine-gained in late.
Regards
Bibo Mao
RegardsThanks,
Bibo Mao
+
+ 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(®_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, ®_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;
Zixing