linux-kernel - Re: [PATCH 2/2] KVM: vmx/pmu: Clear DEBUGCTLMSR

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Date:   Tue, 23 Feb 2021 08:38:29 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu@...ux.intel.com>
Cc:     Paolo Bonzini <pbonzini@...hat.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,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: vmx/pmu: Clear DEBUGCTLMSR_LBR bit on the debug
 breakpoint event

On Tue, Feb 23, 2021, Like Xu wrote:
> When the processor that support model-specific LBR generates a debug
> breakpoint event, it automatically clears the LBR flag. This action
> does not clear previously stored LBR stack MSRs. (Intel SDM 17.4.2)
> 
> Signed-off-by: Like Xu <like.xu@...ux.intel.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index e0a3a9be654b..4951b535eb7f 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -4795,6 +4795,7 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>  	u32 intr_info, ex_no, error_code;
>  	unsigned long cr2, rip, dr6;
>  	u32 vect_info;
> +	u64 lbr_ctl;
>  
>  	vect_info = vmx->idt_vectoring_info;
>  	intr_info = vmx_get_intr_info(vcpu);
> @@ -4886,6 +4887,10 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>  		rip = kvm_rip_read(vcpu);
>  		kvm_run->debug.arch.pc = vmcs_readl(GUEST_CS_BASE) + rip;
>  		kvm_run->debug.arch.exception = ex_no;
> +		/* On the debug breakpoint event, the LBREn bit is cleared. */

Except this code is in BP_VECTOR, not DB_VECTOR as it should be.

  When the processor generates a debug exception (#DB), it automatically clears
  the LBR flag before executing the exception handler. This action does not
  clear previously stored LBR stack MSRs.

> +		lbr_ctl = vmcs_read64(GUEST_IA32_DEBUGCTL);
> +		if (lbr_ctl & DEBUGCTLMSR_LBR)
> +			vmcs_write64(GUEST_IA32_DEBUGCTL, lbr_ctl & ~DEBUGCTLMSR_LBR);
>  		break;
>  	case AC_VECTOR:
>  		if (guest_inject_ac(vcpu)) {
> -- 
> 2.29.2
>