Re: [PATCH 2/2] KVM: x86: Mask LVTPC when handling a PMI

[Mingwei Zhang <mizhang@google.com>]

On Fri, Sep 01, 2023, Jim Mattson wrote:
> Per the SDM, "When the local APIC handles a performance-monitoring
> counters interrupt, it automatically sets the mask flag in the LVT
> performance counter register."
> 
> Add this behavior to KVM's local APIC emulation, to reduce the
> incidence of "dazed and confused" spurious NMI warnings in Linux
> guests (at least, those that use a PMI handler with "late_ack").
> 
> Fixes: 23930f9521c9 ("KVM: x86: Enable NMI Watchdog via in-kernel PIT source")
> Signed-off-by: Jim Mattson <jmattson@google.com>
Tested-by: Mingwei Zhang <mizhang@google.com>

I see consistent number of PMIs and NMIs when running perf on an idle
VM.
> ---
>  arch/x86/kvm/lapic.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index a983a16163b1..1a79ec54ae1e 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2743,6 +2743,8 @@ int kvm_apic_local_deliver(struct kvm_lapic *apic, int lvt_type)
>  		vector = reg & APIC_VECTOR_MASK;
>  		mode = reg & APIC_MODE_MASK;
>  		trig_mode = reg & APIC_LVT_LEVEL_TRIGGER;
> +		if (lvt_type == APIC_LVTPC)
> +			kvm_lapic_set_reg(apic, lvt_type, reg | APIC_LVT_MASKED);
>  		return __apic_accept_irq(apic, mode, vector, 1, trig_mode,
>  					NULL);
>  	}
> -- 
> 2.42.0.283.g2d96d420d3-goog
>