Re: [PATCH v2 13/54] perf: core/x86: Forbid PMI handler when guest own PMU

Zhang, Xiong Y
2024-05-09T07:39:28Z

On 5/7/2024 5:33 PM, Peter Zijlstra wrote:
> On Mon, May 06, 2024 at 05:29:38AM +0000, Mingwei Zhang wrote:
>
>> @@ -1749,6 +1749,23 @@ perf_event_nmi_handler(unsigned int cmd, struct pt_regs *regs)
>> u64 finish_clock;
>> int ret;
>>
>> + /*
>> + * When guest pmu context is loaded this handler should be forbidden from
>> + * running, the reasons are:
>> + * 1. After x86_perf_guest_enter() is called, and before cpu enter into
>> + * non-root mode, NMI could happen, but x86_pmu_handle_irq() restore PMU
>> + * to use NMI vector, which destroy KVM PMI vector setting.
>> + * 2. When VM is running, host NMI other than PMI causes VM exit, KVM will
>> + * call host NMI handler (vmx_vcpu_enter_exit()) first before KVM save
>> + * guest PMU context (kvm_pmu_save_pmu_context()), as x86_pmu_handle_irq()
>> + * clear global_status MSR which has guest status now, then this destroy
>> + * guest PMU status.
>> + * 3. After VM exit, but before KVM save guest PMU context, host NMI other
>> + * than PMI could happen, x86_pmu_handle_irq() clear global_status MSR
>> + * which has guest status now, then this destroy guest PMU status.
>> + */
>> + if (perf_is_guest_context_loaded())
>> + return 0;
>
> A function call makes sense because? Also, isn't this naming at least the purpose of function call is to re-use the per-cpu variable defined in
perf core, otherwise another per-cpu variable will be defined in
arch/x86/event/core.c, whether function call or per-cpu variable depends on
the interface between perf and KVM.
> very little misleading? Specifically this is about passthrough, not
> guest context per se.
>
>> /*
>> * All PMUs/events that share this PMI handler should make sure to
>> * increment active_events for their events.
>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>> index acf16676401a..5da7de42954e 100644
>> --- a/include/linux/perf_event.h
>> +++ b/include/linux/perf_event.h
>> @@ -1736,6 +1736,7 @@ extern int perf_get_mediated_pmu(void);
>> extern void perf_put_mediated_pmu(void);
>> void perf_guest_enter(void);
>> void perf_guest_exit(void);
>> +bool perf_is_guest_context_loaded(void);
>> #else /* !CONFIG_PERF_EVENTS: */
>> static inline void *
>> perf_aux_output_begin(struct perf_output_handle *handle,
>> @@ -1830,6 +1831,10 @@ static inline int perf_get_mediated_pmu(void)
>> static inline void perf_put_mediated_pmu(void) { }
>> static inline void perf_guest_enter(void) { }
>> static inline void perf_guest_exit(void) { }
>> +static inline bool perf_is_guest_context_loaded(void)
>> +{
>> + return false;
>> +}
>> #endif
>>
>> #if defined(CONFIG_PERF_EVENTS) && defined(CONFIG_CPU_SUP_INTEL)
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 4c6daf5cc923..184d06c23391 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -5895,6 +5895,11 @@ void perf_guest_exit(void)
>> perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
>> }
>>
>> +bool perf_is_guest_context_loaded(void)
>> +{
>> + return __this_cpu_read(perf_in_guest);
>> +}
>> +
>> /*
>> * Holding the top-level event's child_mutex means that any
>> * descendant process that has inherited this event will block
>> --
>> 2.45.0.rc1.225.g2a3ae87e7f-goog
>>
>