linux-kernel - Re: [PATCH 3/5] KVM: VMX: RTIT_CTL_BRANCH_EN has no dependency on other CPUID bit

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Date:   Wed, 25 Aug 2021 11:30:49 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>,
        "Alexander Shishkin (hwtracing + intel_th + stm + R:perf)" 
        <alexander.shishkin@...ux.intel.com>
Cc:     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, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 3/5] KVM: VMX: RTIT_CTL_BRANCH_EN has no dependency on
 other CPUID bit

+Alexander

On 24/8/2021 7:07 pm, Xiaoyao Li wrote:
> Per Intel SDM, RTIT_CTL_BRANCH_EN bit has no dependency on any CPUID
> leaf 0x14.
> 
> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
> ---
>   arch/x86/kvm/vmx/vmx.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 7ed96c460661..4a70a6d2f442 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7116,7 +7116,8 @@ static void update_intel_pt_cfg(struct kvm_vcpu *vcpu)
>   
>   	/* Initialize and clear the no dependency bits */
>   	vmx->pt_desc.ctl_bitmask = ~(RTIT_CTL_TRACEEN | RTIT_CTL_OS |
> -			RTIT_CTL_USR | RTIT_CTL_TSC_EN | RTIT_CTL_DISRETC);
> +			RTIT_CTL_USR | RTIT_CTL_TSC_EN | RTIT_CTL_DISRETC |
> +			RTIT_CTL_BRANCH_EN);
>   
>   	/*
>   	 * If CPUID.(EAX=14H,ECX=0):EBX[0]=1 CR3Filter can be set otherwise
> @@ -7134,12 +7135,11 @@ static void update_intel_pt_cfg(struct kvm_vcpu *vcpu)
>   				RTIT_CTL_CYC_THRESH | RTIT_CTL_PSB_FREQ);
>   
>   	/*
> -	 * If CPUID.(EAX=14H,ECX=0):EBX[3]=1 MTCEn BranchEn and
> -	 * MTCFreq can be set
> +	 * If CPUID.(EAX=14H,ECX=0):EBX[3]=1 MTCEn and MTCFreq can be set

If CPUID.(EAX=14H,ECX=0):EBX[3]=1,

	"indicates support of MTC timing packet and suppression of COFI-based packets."

Per 31.2.5.4 Branch Enable (BranchEn),

	"If BranchEn is not set, then relevant COFI packets (TNT, TIP*, FUP, MODE.*) 
are suppressed."

I think if the COFI capability is suppressed, the software can't set the 
BranchEn bit, right ?

>   	 */
>   	if (intel_pt_validate_cap(vmx->pt_desc.caps, PT_CAP_mtc))
>   		vmx->pt_desc.ctl_bitmask &= ~(RTIT_CTL_MTC_EN |
> -				RTIT_CTL_BRANCH_EN | RTIT_CTL_MTC_RANGE);
> +					      RTIT_CTL_MTC_RANGE);
>   
>   	/* If CPUID.(EAX=14H,ECX=0):EBX[4]=1 FUPonPTW and PTWEn can be set */
>   	if (intel_pt_validate_cap(vmx->pt_desc.caps, PT_CAP_ptwrite))
>