linux-kernel - Re: [PATCH v2 12/15] KVM: x86/vmx: Disable Arch LBREn bit in #DB and warm reset

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Message-ID: <Y9RLowPBbQFCqLRf@google.com>
Date:   Fri, 27 Jan 2023 22:09:39 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     pbonzini@...hat.com, jmattson@...gle.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, like.xu.linux@...il.com,
        kan.liang@...ux.intel.com, wei.w.wang@...el.com
Subject: Re: [PATCH v2 12/15] KVM: x86/vmx: Disable Arch LBREn bit in #DB and
 warm reset

On Thu, Nov 24, 2022, Yang Weijiang wrote:
> Per SDM 3B, Chapter 18:
> “On a debug breakpoint event (#DB), IA32_LBR_CTL.LBREn is cleared.”
> and "On a warm reset, all LBR MSRs, including IA32_LBR_DEPTH, have their
> values preserved. However, IA32_LBR_CTL.LBREn is cleared to 0, disabling
> LBRs.", clear the bit manually before inject #DB or when vCPU is in warm
> reset.
> 
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 24 +++++++++++++++++++++++-
>  1 file changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 3bc892e8cf7a..6ad765ea4059 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1695,6 +1695,20 @@ static void vmx_clear_hlt(struct kvm_vcpu *vcpu)
>  		vmcs_write32(GUEST_ACTIVITY_STATE, GUEST_ACTIVITY_ACTIVE);
>  }
>  
> +static void disable_arch_lbr_ctl(struct kvm_vcpu *vcpu)
> +{
> +	struct lbr_desc *lbr_desc = vcpu_to_lbr_desc(vcpu);
> +	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
> +
> +	if (kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR) &&
> +	    test_bit(INTEL_PMC_IDX_FIXED_VLBR, pmu->pmc_in_use) &&
> +	    lbr_desc->event) {

I don't see any reason to check that an event is actually assigned.  The behavior
is architectural, whether or not KVM is actively exposing LBRs to the guest is
irrelevant.

> +		u64 ctl = vmcs_read64(GUEST_IA32_LBR_CTL);
> +
> +		vmcs_write64(GUEST_IA32_LBR_CTL, ctl & ~ARCH_LBR_CTL_LBREN);
> +	}
> +}
> +
>  static void vmx_inject_exception(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_queued_exception *ex = &vcpu->arch.exception;
> @@ -1738,6 +1752,9 @@ static void vmx_inject_exception(struct kvm_vcpu *vcpu)
>  	vmcs_write32(VM_ENTRY_INTR_INFO_FIELD, intr_info);
>  
>  	vmx_clear_hlt(vcpu);
> +
> +	if (ex->vector == DB_VECTOR)
> +		disable_arch_lbr_ctl(vcpu);
>  }
>  
>  static void vmx_setup_uret_msr(struct vcpu_vmx *vmx, unsigned int msr,
> @@ -4796,7 +4813,9 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  
>  	vmx_update_fb_clear_dis(vcpu, vmx);
>  
> -	if (!init_event && cpu_has_vmx_arch_lbr())
> +	if (init_event)

INIT and warm RESET are not the same thing, i.e. this is flat out wrong.

> +		disable_arch_lbr_ctl(vcpu);
> +	else if (cpu_has_vmx_arch_lbr())
>  		vmcs_write64(GUEST_IA32_LBR_CTL, 0);
>  }
>  
> @@ -4873,6 +4892,9 @@ static void vmx_inject_nmi(struct kvm_vcpu *vcpu)
>  			INTR_TYPE_NMI_INTR | INTR_INFO_VALID_MASK | NMI_VECTOR);
>  
>  	vmx_clear_hlt(vcpu);
> +
> +	if (vcpu->arch.exception.vector == DB_VECTOR)

Huh?  This is _very_ obviously injecting NMIs, not #DBs.

> +		disable_arch_lbr_ctl(vcpu);
>  }
>  
>  bool vmx_get_nmi_mask(struct kvm_vcpu *vcpu)
> -- 
> 2.27.0
>