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

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Message-ID: <4b504274-4241-0e3e-3ed3-7804b72b7ee8@loongson.cn>
Date: Mon, 26 Jan 2026 10:09:47 +0800
From: Bibo Mao <maobibo@...ngson.cn>
To: Zixing Liu <liushuyu@...c.io>, WANG Xuerui <kernel@...0n.name>,
 Huacai Chen <chenhuacai@...nel.org>
Cc: 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>, 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: Re: [PATCH 1/1] KVM: Add KVM_GET_REG_LIST ioctl for LoongArch



On 2026/1/25 下午1:43, Zixing Liu wrote:
> 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;
> +	}
CSR_MAX_NUMS is max number of accessible CSR registers, instead only 
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.

Regards
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(&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;
>