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

[Sean Christopherson <seanjc@google.com>]

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

>  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.