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

[Sean Christopherson <seanjc@google.com>]

On Mon, Sep 25, 2023, Mingwei Zhang wrote:
> From: Jim Mattson <jmattson@google.com>
> 
> 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")

This Fixes is wrong.  Prior to commit f5132b01386b ("KVM: Expose a version 2
architectural PMU to a guests"), KVM didn't ever deliver interrupts via the LVTPC
entry.  E.g. prior to that commit, the only reference to APIC_LVTPC is in
kvm_lapic_reg_write:

  arch/x86/kvm $ git grep APIC_LVTPC f5132b01386b^
  f5132b01386b^:lapic.c:  case APIC_LVTPC:

Commit 23930f9521c9 definitely set the PMU support up to fail, but the bug would
never have existed if kvm_deliver_pmi() had been written as:

void kvm_deliver_pmi(struct kvm_vcpu *vcpu)
{
	struct kvm_lapic *apic = vcpu->arch.apic;

	if (apic && kvm_apic_local_deliver(apic, APIC_LVTPC))
		kvm_lapic_set_reg(apic, APIC_LVTPC,
				  kvm_lapic_get_reg(apic, LVTPC) | APIC_LVT_MASKED);
}

And this needs an explicit Cc: to stable because KVM opts out of AUTOSEL.

So

  Fixes: f5132b01386b ("KVM: Expose a version 2 architectural PMU to a guests")
  Cc: stable@vger.kernel.org

> Signed-off-by: Jim Mattson <jmattson@google.com>
> Tested-by: Mingwei Zhang <mizhang@google.com>

When posting patches on behalf of others, you need to provide your SoB.

> ---
>  arch/x86/kvm/lapic.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 113ca9661ab2..1f3d56a1f45f 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2729,13 +2729,17 @@ int kvm_apic_local_deliver(struct kvm_lapic *apic, int lvt_type)
>  {
>  	u32 reg = kvm_lapic_get_reg(apic, lvt_type);
>  	int vector, mode, trig_mode;
> +	int r;
>  
>  	if (kvm_apic_hw_enabled(apic) && !(reg & APIC_LVT_MASKED)) {
>  		vector = reg & APIC_VECTOR_MASK;
>  		mode = reg & APIC_MODE_MASK;
>  		trig_mode = reg & APIC_LVT_LEVEL_TRIGGER;
> -		return __apic_accept_irq(apic, mode, vector, 1, trig_mode,
> -					NULL);
> +
> +		r = __apic_accept_irq(apic, mode, vector, 1, trig_mode, NULL);
> +		if (r && lvt_type == APIC_LVTPC)
> +			kvm_lapic_set_reg(apic, lvt_type, reg | APIC_LVT_MASKED);

Belated feedback, I think I'd prefer to write this as

			kvm_lapic_set_reg(apic, APIC_LVTPC, reg | APIC_LVT_MASKED);

so that this code will show up when searching for APIC_LVTPC.

> +		return r;
>  	}
>  	return 0;
>  }
> -- 
> 2.42.0.515.g380fc7ccd1-goog
>