linux-kernel - Re: [PATCH v10 15/16] KVM: x86: Add Arch LBR data MSR access interface

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Date:   Thu, 28 Apr 2022 11:05:29 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Yang Weijiang <weijiang.yang@...el.com>, pbonzini@...hat.com,
        jmattson@...gle.com, seanjc@...gle.com, like.xu.linux@...il.com,
        vkuznets@...hat.com, wei.w.wang@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 15/16] KVM: x86: Add Arch LBR data MSR access
 interface



On 4/22/2022 3:55 AM, Yang Weijiang wrote:
> Arch LBR MSRs are xsave-supported, but they're operated as "independent"
> xsave feature by PMU code, i.e., during thread/process context switch,
> the MSRs are saved/restored with PMU specific code instead of generic
> kernel fpu XSAVES/XRSTORS operation. 

During thread/process context switch, Linux perf still uses the 
XSAVES/XRSTORS operation to save/restore the LBR MSRs.

Linux perf only manipulates these MSRs only when the xsave feature is 
not supported.

Thanks,
Kan

> When vcpu guest/host fpu state swap
> happens, Arch LBR MSRs won't be touched so access them directly.
> 
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> ---
>   arch/x86/kvm/vmx/pmu_intel.c | 10 ++++++++++
>   1 file changed, 10 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index 79eecbffa07b..5f81644c4612 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -431,6 +431,11 @@ static int intel_pmu_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   	case MSR_ARCH_LBR_CTL:
>   		msr_info->data = vmcs_read64(GUEST_IA32_LBR_CTL);
>   		return 0;
> +	case MSR_ARCH_LBR_FROM_0 ... MSR_ARCH_LBR_FROM_0 + 31:
> +	case MSR_ARCH_LBR_TO_0 ... MSR_ARCH_LBR_TO_0 + 31:
> +	case MSR_ARCH_LBR_INFO_0 ... MSR_ARCH_LBR_INFO_0 + 31:
> +		rdmsrl(msr_info->index, msr_info->data);
> +		return 0;
>   	default:
>   		if ((pmc = get_gp_pmc(pmu, msr, MSR_IA32_PERFCTR0)) ||
>   		    (pmc = get_gp_pmc(pmu, msr, MSR_IA32_PMC0))) {
> @@ -512,6 +517,11 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   		    (data & ARCH_LBR_CTL_LBREN))
>   			intel_pmu_create_guest_lbr_event(vcpu);
>   		return 0;
> +	case MSR_ARCH_LBR_FROM_0 ... MSR_ARCH_LBR_FROM_0 + 31:
> +	case MSR_ARCH_LBR_TO_0 ... MSR_ARCH_LBR_TO_0 + 31:
> +	case MSR_ARCH_LBR_INFO_0 ... MSR_ARCH_LBR_INFO_0 + 31:
> +		wrmsrl(msr_info->index, msr_info->data);
> +		return 0;
>   	default:
>   		if ((pmc = get_gp_pmc(pmu, msr, MSR_IA32_PERFCTR0)) ||
>   		    (pmc = get_gp_pmc(pmu, msr, MSR_IA32_PMC0))) {