linux-kernel - Re: [PATCH 1/2] KVM: vmx/pmu: Fix dummy check if lbr

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Date:   Tue, 23 Feb 2021 09:15:12 -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 1/2] KVM: vmx/pmu: Fix dummy check if lbr_desc->event is
 created

On Tue, Feb 23, 2021, Like Xu wrote:
> If lbr_desc->event is successfully created, the intel_pmu_create_
> guest_lbr_event() will return 0, otherwise it will return -ENOENT,
> and then jump to LBR msrs dummy handling.
> 
> Fixes: 1b5ac3226a1a ("KVM: vmx/pmu: Pass-through LBR msrs when the guest LBR event is ACTIVE")
> Signed-off-by: Like Xu <like.xu@...ux.intel.com>
> ---
>  arch/x86/kvm/vmx/pmu_intel.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index d1df618cb7de..d6a5fe19ff09 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -320,7 +320,7 @@ static bool intel_pmu_handle_lbr_msrs_access(struct kvm_vcpu *vcpu,
>  	if (!intel_pmu_is_valid_lbr_msr(vcpu, index))
>  		return false;
>  
> -	if (!lbr_desc->event && !intel_pmu_create_guest_lbr_event(vcpu))
> +	if (!lbr_desc->event && intel_pmu_create_guest_lbr_event(vcpu))
>  		goto dummy;

Wouldn't it be better to create an event only on write?  And really, why create
the event in this flow in the first place?  In normal operation, can't event
creation be deferred until GUEST_IA32_DEBUGCTL.DEBUGCTLMSR_LBR=1?  If event
creation fails in that flow, I would think KVM would do its best to create an
event in future runs without waiting for additional actions from the guest.

Also, this bug suggests there's a big gaping hole in the test coverage.  AFAICT,
event contention would lead to a #GP crash in the host due to lbr_desc->event
being dereferenced, no?

>  
>  	/*
> -- 
> 2.29.2
>