Re: [PATCH v2 36/54] KVM: x86/pmu: Switch PMI handler at KVM context switch boundary

Sandipan Das
2024-07-10T08:37:35Z

On 5/6/2024 11:00 AM, Mingwei Zhang wrote:
> From: Xiong Zhang <xiong.y.zhang@linux.intel.com>
>
> Switch PMI handler at KVM context switch boundary because KVM uses a
> separate maskable interrupt vector other than the NMI handler for the host
> PMU to process its own PMIs. So invoke the perf API that allows
> registration of the PMI handler.
>
> Signed-off-by: Xiong Zhang <xiong.y.zhang@linux.intel.com>
> Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
> ---
> arch/x86/kvm/pmu.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index 2ad71020a2c0..a12012a00c11 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -1097,6 +1097,8 @@ void kvm_pmu_save_pmu_context(struct kvm_vcpu *vcpu)
> if (pmc->counter)
> wrmsrl(pmc->msr_counter, 0);
> }
> +
> + x86_perf_guest_exit();
> }
>
> void kvm_pmu_restore_pmu_context(struct kvm_vcpu *vcpu)
> @@ -1107,6 +1109,8 @@ void kvm_pmu_restore_pmu_context(struct kvm_vcpu *vcpu)
>
> lockdep_assert_irqs_disabled();
>
> + x86_perf_guest_enter(kvm_lapic_get_reg(vcpu->arch.apic, APIC_LVTPC));
> +

Reading the LVTPC for a vCPU that does not have a struct kvm_lapic allocated
leads to a NULL pointer dereference. I noticed this while trying to run a
minimalistic guest like https://github.com/dpw/kvm-hello-world

Does this require a kvm_lapic_enabled() or similar check?

> static_call_cond(kvm_x86_pmu_restore_pmu_context)(vcpu);
>
> /*