linux-kernel - Re: [PATCH] KVM: Ignore LBR MSRs with no effect

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Date:   Tue, 19 Feb 2019 14:38:54 +0800
From:   Like Xu <like.xu@...ux.intel.com>
To:     Anton Kuchin <antonkuchin@...dex-team.ru>, kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Evgeny Yakovlev <wrfsh@...dex-team.ru>
Subject: Re: [PATCH] KVM: Ignore LBR MSRs with no effect

On 2019/1/29 1:43, Anton Kuchin wrote:
> Win10 attempts to save these registers during KiSaveDebugRegisterState
> if LBR or BTF bits are set in MSR_IA32_DEBUGCTLMSR. It uses DR7 GE and LE
> flags for per-thread switching of these these features so zero value that
> is returned for MSR_IA32_DEBUGCTLMSR has no effect.
> 
> These registers are used for debugging and shouldn't cause #GP and
> guest crash so just return zeroes just like we do for common x86 LBR
> MSRs (DEBUGCTLMSR, LAST[BRANCH|INT][TO|FROM]IP).
> 
> Signed-off-by: Anton Kuchin <antonkuchin@...dex-team.ru>
> ---
>   arch/x86/kvm/vmx/vmx.c | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index f6915f10e584..8bc56cf027ed 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1769,6 +1769,13 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   		else
>   			msr_info->data = vmx->pt_desc.guest.addr_a[index / 2];
>   		break;
> +	case MSR_LBR_TOS:
> +	case MSR_LBR_NHM_FROM ... MSR_LBR_NHM_FROM + 31:
> +	case MSR_LBR_NHM_TO ... MSR_LBR_NHM_TO + 31:
> +	case MSR_LBR_CORE_FROM ... MSR_LBR_CORE_FROM + 7:
> +	case MSR_LBR_CORE_TO ... MSR_LBR_CORE_TO + 7:
> +		msr_info->data = 0;
> +		break;

It's better to move LBR stack MSRs set-up to 
vmx/pmu_intel.c:intel_pmu_get_msr and using specified sizes (31 or 7) is 
not a good practice.

vLBR is still in an unenabled state and
please check https://lkml.org/lkml/2018/12/26/82 for dependency check.

>   	case MSR_TSC_AUX:
>   		if (!msr_info->host_initiated &&
>   		    !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
>