Re: [PATCH v4 11/38] perf/x86: Forbid PMI handler when guest own PMU

["Mi, Dapeng" <dapeng1.mi@linux.intel.com>]

On 5/15/2025 8:00 AM, Sean Christopherson wrote:
> On Mon, Mar 24, 2025, Mingwei Zhang wrote:
>> If a guest PMI is delivered after VM-exit, the KVM maskable interrupt will
>> be held pending until EFLAGS.IF is set. In the meantime, if the logical
>> processor receives an NMI for any reason at all, perf_event_nmi_handler()
>> will be invoked. If there is any active perf event anywhere on the system,
>> x86_pmu_handle_irq() will be invoked, and it will clear
>> IA32_PERF_GLOBAL_STATUS. By the time KVM's PMI handler is invoked, it will
>> be a mystery which counter(s) overflowed.
>>
>> When LVTPC is using KVM PMI vecotr, PMU is owned by guest, Host NMI let
>> x86_pmu_handle_irq() run, x86_pmu_handle_irq() restore PMU vector to NMI
>> and clear IA32_PERF_GLOBAL_STATUS, this breaks guest vPMU passthrough
>> environment.
>>
>> So modify perf_event_nmi_handler() to check perf_in_guest per cpu variable,
>> and if so, to simply return without calling x86_pmu_handle_irq().
>>
>> Suggested-by: Jim Mattson <jmattson@google.com>
>> Signed-off-by: Mingwei Zhang <mizhang@google.com>
>> Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
>> ---
>>  arch/x86/events/core.c | 27 +++++++++++++++++++++++++--
>>  1 file changed, 25 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
>> index 28161d6ff26d..96a173bbbec2 100644
>> --- a/arch/x86/events/core.c
>> +++ b/arch/x86/events/core.c
>> @@ -54,6 +54,8 @@ DEFINE_PER_CPU(struct cpu_hw_events, cpu_hw_events) = {
>>  	.pmu = &pmu,
>>  };
>>  
>> +static DEFINE_PER_CPU(bool, pmi_vector_is_nmi) = true;
> I strongly prefer guest_ctx_loaded.  pmi_vector_is_nmi very inflexible and
> doesn't communicate *why* perf's NMI handler needs to ignore NMIs

Sure.


>
>>  DEFINE_STATIC_KEY_FALSE(rdpmc_never_available_key);
>>  DEFINE_STATIC_KEY_FALSE(rdpmc_always_available_key);
>>  DEFINE_STATIC_KEY_FALSE(perf_is_hybrid);
>> @@ -1737,6 +1739,24 @@ 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 perf_guest_enter() is called, and before cpu enter into
>> +	 *    non-root mode, host non-PMI 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 non-PMI NMI causes VM exit, KVM will
>> +	 *    call host NMI handler (vmx_vcpu_enter_exit()) first before KVM save
>> +	 *    guest PMU context (kvm_pmu_put_guest_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 non-PMI NMI
>> +	 *    could happen, x86_pmu_handle_irq() clear global_status MSR which has
>> +	 *    guest status now, then this destroy guest PMU status.
>> +	 */
> This *might* be useful for a changelog, but even then it's probably overkill.
> NMIs can happen at any time, that's the full the story.  Enumerating the exact
> edge cases adds a lot of noise and not much value.

OK, we just want it to be understood more easily. :)


>