Re: [RFC PATCH 07/41] perf/x86: Add interface to reflect virtual LVTPC_MASK bit onto HW

[Sean Christopherson]

On Fri, Jan 26, 2024, Xiong Zhang wrote:
> From: Xiong Zhang <xiong.y.zhang@xxxxxxxxx>
> 
> When guest clear LVTPC_MASK bit in guest PMI handler at PMU passthrough
> mode, this bit should be reflected onto HW, otherwise HW couldn't generate
> PMI again during VM running until it is cleared.

This fixes a bug in the previous patch, i.e. this should not be a standalone
patch.

> 
> This commit set HW LVTPC_MASK bit at PMU vecctor switching to KVM PMI
> vector.
> 
> Signed-off-by: Xiong Zhang <xiong.y.zhang@xxxxxxxxx>
> Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> ---
>  arch/x86/events/core.c            | 9 +++++++--
>  arch/x86/include/asm/perf_event.h | 2 +-
>  arch/x86/kvm/lapic.h              | 1 -
>  3 files changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index 3f87894d8c8e..ece042cfb470 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -709,13 +709,18 @@ void perf_guest_switch_to_host_pmi_vector(void)
>  }
>  EXPORT_SYMBOL_GPL(perf_guest_switch_to_host_pmi_vector);
>  
> -void perf_guest_switch_to_kvm_pmi_vector(void)
> +void perf_guest_switch_to_kvm_pmi_vector(bool mask)
>  {
>  	lockdep_assert_irqs_disabled();
>  
> -	apic_write(APIC_LVTPC, APIC_DM_FIXED | KVM_VPMU_VECTOR);
> +	if (mask)
> +		apic_write(APIC_LVTPC, APIC_DM_FIXED | KVM_VPMU_VECTOR |
> +			   APIC_LVT_MASKED);
> +	else
> +		apic_write(APIC_LVTPC, APIC_DM_FIXED | KVM_VPMU_VECTOR);
>  }

Or more simply:

void perf_guest_enter(u32 guest_lvtpc)
{
	...

	apic_write(APIC_LVTPC, APIC_DM_FIXED | KVM_VPMU_VECTOR |
			       (guest_lvtpc & APIC_LVT_MASKED));
}

and then on the KVM side:

	perf_guest_enter(kvm_lapic_get_reg(vcpu->arch.apic, APIC_LVTPC));

because an in-kernel APIC should be a hard requirement for the mediated PMU.