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Message-ID: <f4ba7538-e413-d8c8-d4b6-7c8686c4aec6@loongson.cn>
Date: Tue, 10 Feb 2026 11:43:52 +0800
From: Bibo Mao <maobibo@...ngson.cn>
To: liushuyu <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>, 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 v5] KVM: Add KVM_GET_REG_LIST ioctl for LoongArch
On 2026/2/10 上午11:23, liushuyu wrote:
> Hi Bibo,
>
>>
>>
>> On 2026/2/5 下午1:18, 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@...c.io>
>>> ---
>>> Documentation/virt/kvm/api.rst | 2 +-
>>> arch/loongarch/kvm/vcpu.c | 120 +++++++++++++++++++++++++++++++++
>>> 2 files changed, 121 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..de02e409ae39 100644
>>> --- a/arch/loongarch/kvm/vcpu.c
>>> +++ b/arch/loongarch/kvm/vcpu.c
>>> @@ -5,6 +5,7 @@
>>> #include <linux/kvm_host.h>
>>> #include <asm/fpu.h>
>>> +#include <asm/kvm_host.h>
>>> #include <asm/lbt.h>
>>> #include <asm/loongarch.h>
>>> #include <asm/setup.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 +1189,105 @@ 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;
>>> + const unsigned int csrs_to_save[] = {
>>> + LOONGARCH_CSR_CRMD, LOONGARCH_CSR_PRMD,
>>> + LOONGARCH_CSR_EUEN, LOONGARCH_CSR_MISC,
>>> + LOONGARCH_CSR_ECFG, LOONGARCH_CSR_ESTAT,
>>> + LOONGARCH_CSR_ERA, LOONGARCH_CSR_BADV,
>>> + LOONGARCH_CSR_BADI, LOONGARCH_CSR_EENTRY,
>>> + LOONGARCH_CSR_TLBIDX, LOONGARCH_CSR_TLBEHI,
>>> + LOONGARCH_CSR_TLBELO0, LOONGARCH_CSR_TLBELO1,
>>> + LOONGARCH_CSR_ASID, LOONGARCH_CSR_PGDL,
>>> + LOONGARCH_CSR_PGDH, LOONGARCH_CSR_PGD,
>>> + LOONGARCH_CSR_PWCTL0, LOONGARCH_CSR_PWCTL1,
>>> + LOONGARCH_CSR_STLBPGSIZE, LOONGARCH_CSR_RVACFG,
>>> + LOONGARCH_CSR_CPUID, LOONGARCH_CSR_PRCFG1,
>>> + LOONGARCH_CSR_PRCFG2, LOONGARCH_CSR_PRCFG3,
>>> + LOONGARCH_CSR_KS0, LOONGARCH_CSR_KS1,
>>> + LOONGARCH_CSR_KS2, LOONGARCH_CSR_KS3,
>>> + LOONGARCH_CSR_KS4, LOONGARCH_CSR_KS5,
>>> + LOONGARCH_CSR_KS6, LOONGARCH_CSR_KS7,
>>> + LOONGARCH_CSR_TMID, LOONGARCH_CSR_CNTC,
>>> + LOONGARCH_CSR_TINTCLR, LOONGARCH_CSR_LLBCTL,
>>> + LOONGARCH_CSR_IMPCTL1, LOONGARCH_CSR_IMPCTL2,
>>> + LOONGARCH_CSR_TLBRENTRY, LOONGARCH_CSR_TLBRBADV,
>>> + LOONGARCH_CSR_TLBRERA, LOONGARCH_CSR_TLBRSAVE,
>>> + LOONGARCH_CSR_TLBRELO0, LOONGARCH_CSR_TLBRELO1,
>>> + LOONGARCH_CSR_TLBREHI, LOONGARCH_CSR_TLBRPRMD,
>>> + LOONGARCH_CSR_DMWIN0, LOONGARCH_CSR_DMWIN1,
>>> + LOONGARCH_CSR_DMWIN2, LOONGARCH_CSR_DMWIN3,
>>> + LOONGARCH_CSR_TVAL, LOONGARCH_CSR_TCFG,
>>> + };
>> this increases much kernel stack size usage :)
>>
>> Please wait a moment, I am considering how to cleanup code about CSR
>> registers.
> Okay.
>> And KVM_GET_REG_LIST is not so urgent, else there is
>> KVM_read_from_REG_LIST/KVM_write_from_REG_LIST ioctl commands to
>> access registers in batch mode.
>>
> Adding KVM_GET_REG_LIST is not urgent. However, unlike
> KVM_read_from_REG_LIST and KVM_write_from_REG_LIST, KVM_GET_REG_LIST
> serves a different purpose.
>
> The KVM_GET_REG_LIST ioctl lets user-space VMMs determine the number of
> get/set-able registers (via KVM_GET_ONE_REG/KVM_SET_ONE_REG) and their
> register IDs. User-space VMMs use this ioctl so they don't have to
> hardcode a register ID table for each architecture.
In theory it is so, I only know QEMU VMM now, is there other VMMs which
does not use hardcoded register ID for different architecture? Which one
if there is actually. There is some potential issues by my knowledge
such as:
1. How to solve the compatible issue if KVM_GET_REG_LIST does not support?
2. How to solve dependency between registers? Dependency should be
solved in VMM or KVM hypervisr.
Regards
Bibo Mao
>
> I also had trouble finding information on the KVM_read_from_REG_LIST and
> KVM_write_from_REG_LIST ioctl commands. As of commit
> 3d29a326eba82d987a82fd59379d6d668b769965, I could not find any logic or
> identifiers for these two ioctls.
>
> In some cases, KVM_GET_REG_LIST is a prerequisite for using
> KVM_read_from_REG_LIST or KVM_write_from_REG_LIST (assuming they exist),
> as you still need to know the register IDs to use these ioctls.
>
>> Regards
>> Bibo Mao
>>
>>> +
>>> + for (i = 0, count = 0;
>>> + i < sizeof(csrs_to_save) / sizeof(csrs_to_save[0]); i++) {
>>> + const u64 reg = KVM_IOC_CSRID(i);
>>> + if (uindices && put_user(reg, uindices++))
>>> + return -EFAULT;
>>> + count++;
>>> + }
>>> +
>>> + /* Skip PMU CSRs if not supported by the guest */
>>> + if (!kvm_guest_has_pmu(&vcpu->arch))
>>> + return count;
>>> + for (i = LOONGARCH_CSR_PERFCTRL0; i <= LOONGARCH_CSR_PERFCNTR3;
>>> i++) {
>>> + 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 +1353,24 @@ 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;
>>>
>>
> Thanks,
> Zixing
>
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