linux-kernel - Re: [PATCH 2/3] KVM: vPMU: Fill get_msr MSR_CORE_PERF_GLOBAL_OVF

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Message-ID: <58d59149-669f-7990-4f68-05b32ed693b5@gmail.com>
Date:   Fri, 8 Oct 2021 19:02:31 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 2/3] KVM: vPMU: Fill get_msr MSR_CORE_PERF_GLOBAL_OVF_CTRL
 w/ 0

cc Andi,

On 8/10/2021 5:57 pm, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> SDM section 18.2.3 mentioned that:
> 
>    "IA32_PERF_GLOBAL_OVF_CTL MSR allows software to clear overflow indicator(s) of
>     any general-purpose or fixed-function counters via a single WRMSR."
> 
> It is R/W mentioned by SDM, we read this msr on bare-metal during perf testing,
> the value is always 0 for CLX/SKX boxes on hands. Let's fill get_msr
> MSR_CORE_PERF_GLOBAL_OVF_CTRL w/ 0 as hardware behavior.
> 
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
> Btw, xen also fills get_msr MSR_CORE_PERF_GLOBAL_OVF_CTRL 0.
> 
>   arch/x86/kvm/vmx/pmu_intel.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index 10cc4f65c4ef..47260a8563f9 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -365,7 +365,7 @@ static int intel_pmu_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   		msr_info->data = pmu->global_ctrl;
>   		return 0;
>   	case MSR_CORE_PERF_GLOBAL_OVF_CTRL:
> -		msr_info->data = pmu->global_ovf_ctrl;
> +		msr_info->data = 0;

Tested-by: Like Xu <likexu@...cent.com>

Further, better to drop 'u64 global_ovf_ctrl' directly.

>   		return 0;
>   	default:
>   		if ((pmc = get_gp_pmc(pmu, msr, MSR_IA32_PERFCTR0)) ||
>