linux-kernel - Re: [PATCH v12 07/11] KVM: vmx/pmu: Unmask LBR fields in the MSR_IA32

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Message-ID: <1e1c2e15-ac02-b44b-6de9-87530e050855@intel.com>
Date:   Wed, 8 Jul 2020 09:37:32 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        "Xu, Like" <like.xu@...el.com>
Cc:     Like Xu <like.xu@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, ak@...ux.intel.com,
        wei.w.wang@...el.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v12 07/11] KVM: vmx/pmu: Unmask LBR fields in the
 MSR_IA32_DEBUGCTLMSR emualtion

On 7/8/2020 4:21 AM, Sean Christopherson wrote:
> On Sat, Jun 13, 2020 at 05:42:50PM +0800, Xu, Like wrote:
>> On 2020/6/13 17:14, Xiaoyao Li wrote:
>>> On 6/13/2020 4:09 PM, Like Xu wrote:
[...]
>>>> @@ -237,6 +238,9 @@ static int intel_pmu_get_msr(struct kvm_vcpu *vcpu,
>>>> struct msr_data *msr_info)
>>>>                return 1;
>>>>            msr_info->data = vcpu->arch.perf_capabilities;
>>>>            return 0;
>>>> +    case MSR_IA32_DEBUGCTLMSR:
>>>> +        msr_info->data = vmcs_read64(GUEST_IA32_DEBUGCTL);
>>>
>>> Can we put the emulation of MSR_IA32_DEBUGCTLMSR in vmx_{get/set})_msr().
>>> AFAIK, MSR_IA32_DEBUGCTLMSR is not a pure PMU related MSR that there is
>>> bit 2 to enable #DB for bus lock.
>> We already have "case MSR_IA32_DEBUGCTLMSR" handler in the vmx_set_msr()
>> and you may apply you bus lock changes in that handler.
> 
> Hrm, but that'd be weird dependency as vmx_set_msr() would need to check for
> #DB bus lock support but not actually write GUEST_IA32_DEBUGCTL, or we'd end
> up writing it twice when both bus lock and LBR are supported.

Yeah. That's what I concerned as well.

> I don't see anything in the series that takes action on writes to
> MSR_IA32_DEBUGCTLMSR beyond updating the VMCS, i.e. AFAICT there isn't any
> reason to call into the PMU, VMX can simply query vmx_get_perf_capabilities()
> to check if it's legal to enable DEBUGCTLMSR_LBR_MASK.
> 
> A question for both LBR and bus lock: would it make sense to cache the
> guest's value in vcpu_vmx so that querying the guest value doesn't require
> a VMREAD?  I don't have a good feel for how frequently it would be accessed.

Cache the guest's value is OK, even though #DB bus lock bit wouldn't be 
toggled frequently in a normal OS.