Message-ID: <913e8e96-9124-2fbe-bd00-f6122962562a@loongson.cn>
Date: Mon, 26 Jan 2026 17:35:11 +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>, 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 2026/1/26 下午4:54, liushuyu wrote:
> Hi Bibo,
> 
>>
>>
>> On 2026/1/26 下午12:22, liushuyu wrote:
>>> Hi Bibo,
>>>
>>>>
>>>>
>>>> On 2026/1/26 上午11:38, liushuyu wrote:
>>>>> Hi Bibo,
>>>>>>
>>>>>>
>>>>>> 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.
>>>>>>
>>>>> Did you mean we should only return the CSR registers initialized in
>>>>> this
>>>>> 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?
>>>> 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
>>>>
>>>> At the beginning, there is big array with size CSR_MAX_NUMS without
>>>> any category, it can be fine-gained in late.
>>>
>>> Does that mean in the new `kvm_loongarch_copy_reg_indices` function, it
>>> is also okay to embed this big list in there?
>> Firstly I do not know how to use ioctl command KVM_GET_REG_LIST in VMM
>> actually.
>>
>> At the second from the word "determine which (special) registers are
>> get/set-able", I think that it is better to set accessible CSR
>> register number and ID rather than the total number CSR_MAX_NUMS.
>>
> Understood. I will post a v2 patch, and we can continue the discussion
> from there to see if I understood your points correctly.
Remember to add such lines in function kvm_init_gcsr_flag() in the next 
round, since MSGINT CSR registers are missing here.

void kvm_init_gcsr_flag(void) {
+        if (cpu_has_msgint) {
+                set_gcsr_hw_flag(LOONGARCH_CSR_ISR0);
+                set_gcsr_hw_flag(LOONGARCH_CSR_ISR1);
+                set_gcsr_hw_flag(LOONGARCH_CSR_ISR2);
+                set_gcsr_hw_flag(LOONGARCH_CSR_ISR3);
+        }
}

Regards
Bibo Mao
> 
>> +static int kvm_loongarch_num_csr_regs(struct kvm_vcpu *vcpu)
>> +{
>> +    unsigned int i, count;
>> +
>> +    count = 0
>> +    for (i = 0; i < CSR_MAX_NUMS; i++) {
>> +        if (!(get_gcsr_flag(i) & (SW_GCSR | HW_GCSR)))
>> +            continue;
>> +        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_num_csr_regs +
>> 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++) {
>> +                if (!(get_gcsr_flag(i) & (SW_GCSR | HW_GCSR)))
>> +                    continue;
>>          reg = KVM_IOC_CSRID(i);
>>          if (put_user(reg, uindices++))
>>              return -EFAULT;
>>      }
>>
>> Regards
>> Bibo Mao
>>>
>>> Or do you want to extract the list from the `kvm_init_gcsr_flag`
>>> function and refactor both functions to share one single big list
>>> (probably in the kvm_host.h header)?
>>>
>>> Because from what I could understand, your previous messages point to
>>> that we need to use this list to make out what CSR registers to return
>>> to the user space VMMs.
>>
>>>
>>>>
>>>> Regards
>>>> Bibo Mao
>>>>>
>>>>>> 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;
>>>>>>>
>>>>>>
>>>>> Thanks,
>>>>> Zixing
>>>>>
>>>>
>>> Thanks,
>>> Zixing
>>>
>>
> Thanks,
> 
> Zixing
> 

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