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

[Huacai Chen]

On Thu, Oct 8, 2026 at 9:50 AM Bibo Mao <maobibo@xxxxxxxxxxx> 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@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> >> ---
> >>   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.


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
> >>
> >>
>