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Message-ID: <972430be-d2f3-49b6-851b-a057ddfcafec@aosc.io>
Date: Mon, 26 Jan 2026 11:38:41 +0800
From: liushuyu <liushuyu@...c.io>
To: Bibo Mao <maobibo@...ngson.cn>, 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
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?
> 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(®_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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