linux-kernel - Re: [PATCH v12 06/11] KVM: vmx/pmu: Expose LBR to guest via MSR_IA32_PERF

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Date:   Wed, 8 Jul 2020 22:38:02 +0800
From:   "Xu, Like" <like.xu@...el.com>
To:     Andi Kleen <ak@...ux.intel.com>, Like Xu <like.xu@...ux.intel.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, wei.w.wang@...el.com,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v12 06/11] KVM: vmx/pmu: Expose LBR to guest via
 MSR_IA32_PERF_CAPABILITIES

On 2020/7/8 21:36, Andi Kleen wrote:
>> +	/*
>> +	 * As a first step, a guest could only enable LBR feature if its cpu
>> +	 * model is the same as the host because the LBR registers would
>> +	 * be pass-through to the guest and they're model specific.
>> +	 */
>> +	if (boot_cpu_data.x86_model != guest_cpuid_model(vcpu))
>> +		return false;
> Could we relax this in a followon patch? (after this series is merged)
Sure, there would be a follow-on patch to relax this check after it's merged.
>
> It's enough of the perf cap LBR version matches, don't need full model
> number match.
I assume you are referring to the LBR_FMT value in the perf_capabilities.
> This would require a way to configure the LBR version
> from qemu.
Sure, I may propose this configuration in the QEMU community.
>
> This would allow more flexibility, for example migration from
> Icelake to Skylake and vice versa.
Yes, we need this flexibility to cover as many platforms as possible.

Thanks,
Like Xu
>
> -Andi