linux-kernel - Re: [PATCH v2 RESEND] KVM: Add KVM_GET_REG

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Message-ID: <CAAhV-H7g4StjP5fnHVVEyR_xFx9=fg6S9UuHWnPpMV_k=ZVGGw@mail.gmail.com>
Date: Sat, 31 Jan 2026 16:48:58 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Zixing Liu <liushuyu@...c.io>
Cc: WANG Xuerui <kernel@...0n.name>, Bibo Mao <maobibo@...ngson.cn>, 
	Kexy Biscuit <kexybiscuit@...c.io>, Mingcong Bai <jeffbai@...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>, kvm@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, loongarch@...ts.linux.dev, 
	linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v2 RESEND] KVM: Add KVM_GET_REG_LIST ioctl for LoongArch

Hi, Zixing,

On Sat, Jan 31, 2026 at 2:07 PM Zixing Liu <liushuyu@...c.io> wrote:
>
> 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@...c.io>
> ---
>  Documentation/virt/kvm/api.rst |  2 +-
>  arch/loongarch/kvm/vcpu.c      | 85 ++++++++++++++++++++++++++++++++++
>  2 files changed, 86 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..ed11438f4544 100644
> --- a/arch/loongarch/kvm/vcpu.c
> +++ b/arch/loongarch/kvm/vcpu.c
> @@ -1186,6 +1186,73 @@ 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_lbt_regs(void)
> +{
> +       /* +1 for the LBT_FTOP flag (inside arch.fpu) */
> +       return sizeof(struct loongarch_lbt) / sizeof(unsigned long) + 1;
> +}
This function has only one line, I think it is better to embed it into
the caller.

Huacai

> +
> +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 += 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;
> +
> +       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 <= 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 +1318,24 @@ 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
>