Re: [PATCH v4] KVM: Add KVM_GET_REG_LIST ioctl for LoongArch

[Bibo Mao <maobibo@loongson.cn>]

Hi Zixing,

Thanks for doing this.

On 2026/2/4 下午7:36, Zixing Liu 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@aosc.io>
> ---
>   Documentation/virt/kvm/api.rst |  2 +-
>   arch/loongarch/kvm/vcpu.c      | 87 ++++++++++++++++++++++++++++++++++
>   2 files changed, 88 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..bd855ee20ee2 100644
> --- a/arch/loongarch/kvm/vcpu.c
> +++ b/arch/loongarch/kvm/vcpu.c
> @@ -3,6 +3,7 @@
>    * Copyright (C) 2020-2023 Loongson Technology Corporation Limited
>    */
>   
> +#include "asm/kvm_host.h"
Had better put after #include <asm/fpu.h>, and keep alphabetical order.
>   #include <linux/kvm_host.h>
>   #include <asm/fpu.h>
>   #include <asm/lbt.h>
> @@ -14,6 +15,8 @@
>   #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 +1189,72 @@ 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;
> +		if (i >= LOONGARCH_CSR_PERFCTRL0 && i <= LOONGARCH_CSR_PERFCNTR3) {
> +			/* Skip PMU CSRs if not supported by the guest */
> +			if (!kvm_guest_has_pmu(&vcpu->arch))
> +				continue;
> +		}
This is workable, gcsr_flag can be changed with structure, and new 
element "int required_features" added. However it does not matter, it 
can be done in later.

CSR registers relative with msgint feature can be done with this method 
also.

How about debug/watch CSR registers? can it be skipped also?  the same 
MERR CSR registers with LOONGARCH_CSR_MERR*.

The CSR register list difference can be checked with 
kvm_loongarch_get_csr() in qemu VMM, with website
https://gitlab.com/qemu-project/qemu/-/blob/master/target/loongarch/kvm/kvm.c?ref_type=heads

Regards
Bibo Mao
> +		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 +1320,24 @@ 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;
>