Re: [PATCH] LoongArch: KVM: Add hypercall crash support

[Bibo Mao <maobibo@loongson.cn>]

On 2026/10/11 下午10:06, Huacai Chen wrote:
> On Thu, Oct 8, 2026 at 9:50 AM Bibo Mao <maobibo@loongson.cn> wrote:
>>
>>
>>
>> On 2026/10/3 下午12:02, Huacai Chen wrote:
>>> Hi, Bibo,
>>>
>>> On Wed, Sep 9, 2026 at 5:01 PM Bibo Mao <maobibo@loongson.cn> wrote:
>>>>
>>>> Now generic pvpanic is supported in Linux kernel which will notify VMM
>>>> with VM crash event. There is some limitations with pvpanic, such as it
>>>> requires good kernel stack or CPU hardware context and panic() C function
>>>> is in health state.
>>>>
>>>> Sometimes CPU hardware context or kernel/irq stack is corrupted. Hypercall
>>>> code is more direct and low-level crash method, which is useful in
>>>> exception entry with assmble language, here add one hypercall code to
>>>> support VM crash function, so that VMM can capture CPU crash context
>>>> in time.
>>>>
>>>> Signed-off-by: Bibo Mao <maobibo@loongson.cn>
>>>> ---
>>>>    arch/loongarch/include/asm/kvm_host.h      |  1 +
>>>>    arch/loongarch/include/asm/kvm_para.h      | 15 +++++++++++++++
>>>>    arch/loongarch/include/uapi/asm/kvm.h      |  1 +
>>>>    arch/loongarch/include/uapi/asm/kvm_para.h |  1 +
>>>>    arch/loongarch/kvm/exit.c                  | 11 +++++++++++
>>>>    arch/loongarch/kvm/vm.c                    |  3 ++-
>>>>    6 files changed, 31 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/loongarch/include/asm/kvm_host.h b/arch/loongarch/include/asm/kvm_host.h
>>>> index 65d91c3ce313..96f9f1317cad 100644
>>>> --- a/arch/loongarch/include/asm/kvm_host.h
>>>> +++ b/arch/loongarch/include/asm/kvm_host.h
>>>> @@ -168,6 +168,7 @@ enum emulation_result {
>>>>    #define LOONGARCH_PV_FEAT_MASK         (BIT(KVM_FEATURE_IPI) |         \
>>>>                                            BIT(KVM_FEATURE_PREEMPT) |     \
>>>>                                            BIT(KVM_FEATURE_STEAL_TIME) |  \
>>>> +                                        BIT(KVM_FEATURE_CRASH) |       \
>>>>                                            BIT(KVM_FEATURE_USER_HCALL) |  \
>>>>                                            BIT(KVM_FEATURE_VIRT_EXTIOI))
>>>>
>>>> diff --git a/arch/loongarch/include/asm/kvm_para.h b/arch/loongarch/include/asm/kvm_para.h
>>>> index fb17ba0fa101..3b183bb1aaa8 100644
>>>> --- a/arch/loongarch/include/asm/kvm_para.h
>>>> +++ b/arch/loongarch/include/asm/kvm_para.h
>>>> @@ -14,6 +14,7 @@
>>>>    #define KVM_HCALL_CODE_SERVICE         0
>>>>    #define KVM_HCALL_CODE_SWDBG           1
>>>>    #define KVM_HCALL_CODE_USER_SERVICE    2
>>>> +#define KVM_HCALL_CODE_CRASH           3
>>>>
>>>>    #define KVM_HCALL_SERVICE              HYPERCALL_ENCODE(HYPERVISOR_KVM, KVM_HCALL_CODE_SERVICE)
>>>>    #define  KVM_HCALL_FUNC_IPI            1
>>>> @@ -22,6 +23,7 @@
>>>>    #define KVM_HCALL_SWDBG                        HYPERCALL_ENCODE(HYPERVISOR_KVM, KVM_HCALL_CODE_SWDBG)
>>>>
>>>>    #define KVM_HCALL_USER_SERVICE         HYPERCALL_ENCODE(HYPERVISOR_KVM, KVM_HCALL_CODE_USER_SERVICE)
>>>> +#define KVM_HCALL_CRASH                        HYPERCALL_ENCODE(HYPERVISOR_KVM, KVM_HCALL_CODE_CRASH)
>>>>
>>>>    /*
>>>>     * LoongArch hypercall return code
>>>> @@ -161,6 +163,19 @@ static __always_inline long kvm_hypercall5(u64 fid,
>>>>           return ret;
>>>>    }
>>>>
>>>> +static __always_inline long kvm_hypercall_crash(void)
>>>> +{
>>>> +       register long ret asm("a0");
>>>> +
>>>> +       __asm__ __volatile__(
>>>> +               "hvcl "__stringify(KVM_HCALL_CRASH)
>>>> +               : "=r" (ret)
>>>> +               : : "memory"
>>>> +               );
>>>> +
>>>> +       return ret;
>>>> +}
>>> Why this function has no caller?
>> Currently we directly modify guest kernel and hard-code the hypercall
>> instruction to debug. This inline function is only example for howto
>> usage, the normal usage such as checking KVM_FEATURE_USER_HCALL feature
>> + kvm_hypercall_crash() is too complicated in exception entry assemble
>> language.
> There are no examples for KVM_HCALL_SWDBG and KVM_HCALL_USER_SERVICE,
> I think KVM_HCALL_CRASH is no special.
> 
> So if you agree, I will remove this example when applying.
Removinge the example usage with KVM_HCALL_CRASH is fine to me.

Regards
Bibo Mao
> 
> 
> Huacai
> 
>>
>> Regards
>> Bibo Mao
>>>
>>>
>>> Huacai
>>>
>>>> +
>>>>    #ifdef CONFIG_PARAVIRT
>>>>    bool kvm_para_available(void);
>>>>    unsigned int kvm_arch_para_features(void);
>>>> diff --git a/arch/loongarch/include/uapi/asm/kvm.h b/arch/loongarch/include/uapi/asm/kvm.h
>>>> index cd0b5c11ca9c..b222bfc48042 100644
>>>> --- a/arch/loongarch/include/uapi/asm/kvm.h
>>>> +++ b/arch/loongarch/include/uapi/asm/kvm.h
>>>> @@ -106,6 +106,7 @@ struct kvm_fpu {
>>>>    #define  KVM_LOONGARCH_VM_FEAT_PTW             8
>>>>    #define  KVM_LOONGARCH_VM_FEAT_MSGINT          9
>>>>    #define  KVM_LOONGARCH_VM_FEAT_PV_PREEMPT      10
>>>> +#define  KVM_LOONGARCH_VM_FEAT_CRASH           11
>>>>
>>>>    /* Device Control API on vcpu fd */
>>>>    #define KVM_LOONGARCH_VCPU_CPUCFG      0
>>>> diff --git a/arch/loongarch/include/uapi/asm/kvm_para.h b/arch/loongarch/include/uapi/asm/kvm_para.h
>>>> index d28cbcadd276..bf1a0c826ca6 100644
>>>> --- a/arch/loongarch/include/uapi/asm/kvm_para.h
>>>> +++ b/arch/loongarch/include/uapi/asm/kvm_para.h
>>>> @@ -16,6 +16,7 @@
>>>>    #define  KVM_FEATURE_IPI               1
>>>>    #define  KVM_FEATURE_STEAL_TIME                2
>>>>    #define  KVM_FEATURE_PREEMPT           3
>>>> +#define  KVM_FEATURE_CRASH             4
>>>>    /* BIT 24 - 31 are features configurable by user space vmm */
>>>>    #define  KVM_FEATURE_VIRT_EXTIOI       24
>>>>    #define  KVM_FEATURE_USER_HCALL                25
>>>> diff --git a/arch/loongarch/kvm/exit.c b/arch/loongarch/kvm/exit.c
>>>> index 4f58e6e2cf86..3b6daa946c76 100644
>>>> --- a/arch/loongarch/kvm/exit.c
>>>> +++ b/arch/loongarch/kvm/exit.c
>>>> @@ -911,6 +911,17 @@ static int kvm_handle_hypercall(struct kvm_vcpu *vcpu, int ecode)
>>>>                   vcpu->run->hypercall.ret = KVM_HCALL_INVALID_CODE;
>>>>                   ret = RESUME_HOST;
>>>>                   break;
>>>> +       case KVM_HCALL_CRASH:
>>>> +               if (!kvm_guest_has_pv_feature(vcpu, KVM_FEATURE_CRASH)) {
>>>> +                       kvm_write_reg(vcpu, LOONGARCH_GPR_A0, KVM_HCALL_INVALID_CODE);
>>>> +                       break;
>>>> +               }
>>>> +
>>>> +               vcpu->run->exit_reason = KVM_EXIT_SYSTEM_EVENT;
>>>> +               vcpu->run->system_event.type = KVM_SYSTEM_EVENT_CRASH;
>>>> +               vcpu->run->system_event.ndata = 0;
>>>> +               ret = RESUME_HOST;
>>>> +               break;
>>>>           case KVM_HCALL_SWDBG:
>>>>                   /* KVM_HCALL_SWDBG only in effective when SW_BP is enabled */
>>>>                   if (vcpu->guest_debug & KVM_GUESTDBG_SW_BP_MASK) {
>>>> diff --git a/arch/loongarch/kvm/vm.c b/arch/loongarch/kvm/vm.c
>>>> index 6dabb227a732..e5d10ceb5e67 100644
>>>> --- a/arch/loongarch/kvm/vm.c
>>>> +++ b/arch/loongarch/kvm/vm.c
>>>> @@ -49,8 +49,9 @@ static void kvm_vm_init_features(struct kvm *kvm)
>>>>                   kvm->arch.kvm_features |= BIT(KVM_LOONGARCH_VM_FEAT_PMU);
>>>>
>>>>           /* Enable all PV features by default */
>>>> -       kvm->arch.pv_features |= BIT(KVM_FEATURE_IPI);
>>>> +       kvm->arch.pv_features |= BIT(KVM_FEATURE_IPI) | BIT(KVM_FEATURE_CRASH);
>>>>           kvm->arch.kvm_features |= BIT(KVM_LOONGARCH_VM_FEAT_PV_IPI);
>>>> +       kvm->arch.kvm_features |= BIT(KVM_LOONGARCH_VM_FEAT_CRASH);
>>>>           if (kvm_pvtime_supported()) {
>>>>                   kvm->arch.pv_features |= BIT(KVM_FEATURE_PREEMPT);
>>>>                   kvm->arch.pv_features |= BIT(KVM_FEATURE_STEAL_TIME);
>>>>
>>>> base-commit: df2908090cda368b01ff43709f51890076c56157
>>>> --
>>>> 2.39.3
>>>>
>>>>
>>