linux-kernel - Re: [PATCH 3/4] KVM: VMX/pmu: Enable inactive vLBR event in guest LBR MSR emulation

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Message-ID: <2db5fe53-c316-2017-c0dc-74190d6ad9ba@gmail.com>
Date:   Wed, 28 Jun 2023 13:50:46 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Xiong Zhang <xiong.y.zhang@...el.com>
Cc:     seanjc@...gle.com, pbonzini@...hat.com, peterz@...radead.org,
        kan.liang@...ux.intel.com, zhenyuw@...ux.intel.com,
        zhiyuan.lv@...el.com, kvm list <kvm@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] KVM: VMX/pmu: Enable inactive vLBR event in guest LBR
 MSR emulation

On 16/6/2023 7:33 pm, Xiong Zhang wrote:
> vLBR event could be inactive in two case:
> a. host per cpu pinned LBR event occupy LBR when vLBR event is created
> b. vLBR event is preempted by host per cpu pinned LBR event during vm
> exit handler.
> When vLBR event is inactive, guest couldn't access LBR msr, and it is
> forced into error state and is excluded from schedule by perf scheduler.
> So vLBR event couldn't be active through perf scheduler even if host per
> cpu pinned LBR event has released LBR, kvm could enable vLBR event
> proactively, then vLBR event may be active and LBR msr could be passthrough
> into guest.
> 
> Signed-off-by: Xiong Zhang <xiong.y.zhang@...el.com>
> ---
>   arch/x86/kvm/vmx/pmu_intel.c | 11 ++++++++++-
>   1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index 741efe2c497b..5a3ab8c8711b 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -314,7 +314,16 @@ static bool intel_pmu_handle_lbr_msrs_access(struct kvm_vcpu *vcpu,
>   	if (!intel_pmu_is_valid_lbr_msr(vcpu, index))
>   		return false;
>   
> -	if (!lbr_desc->event && intel_pmu_create_guest_lbr_event(vcpu) < 0)
> +	/* vLBR event may be inactive, but physical LBR may be free now.
> +	 * but vLBR event is pinned event, once it is inactive state, perf
> +	 * will force it to error state in merge_sched_in() and exclude it from
> +	 * perf schedule, so even if LBR is free now, vLBR event couldn't be active
> +	 * through perf scheduler and vLBR event could be active through
> +	 * perf_event_enable().
> +	 */
> +	if (lbr_desc->event && (lbr_desc->event->state == PERF_EVENT_STATE_ERROR))
> +		perf_event_enable(lbr_desc->event);

After allowing LBR host/guest coexistence, calls perf_event_enable() for events
here do not always succeed, thus this is not a good call point.

As expected here, any erroneous perf_event is released and reprogrammed in the
kvm_pmu_handle_event(), and the perf status for enabled features are checked
near the atomic_switch_perf_msrs().

> +	else if (!lbr_desc->event && intel_pmu_create_guest_lbr_event(vcpu) < 0)
>   		goto dummy;
>   
>   	/*