linux-kernel - Re: [PATCH v2 2/6] KVM: x86/pmu: Refactoring find_arch_event() to pmc_perf_hw

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Date:   Thu, 9 Dec 2021 20:24:34 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>,
        Like Xu <like.xu.linux@...il.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Like Xu <likexu@...cent.com>
Subject: Re: [PATCH v2 2/6] KVM: x86/pmu: Refactoring find_arch_event() to
 pmc_perf_hw_id()

On 12/9/21 04:52, Jim Mattson wrote:
> Why don't we just use PERF_TYPE_RAW for guest counters all of the
> time? What is the advantage of matching entries in a table so that we
> can use PERF_TYPE_HARDWARE?

In theory it's so that hosts without support for architectural PMU 
events can map the architectural PMU events to their own.  In practice, 
we can probably return early from intel_pmu_refresh if the host does not 
support X86_FEATURE_ARCH_PERFMON, because only the oldest Pentium 4 
toasters^Wprocessors probably have VMX but lack architectural PMU.

Paolo