linux-kernel - Re: [PATCH v11 01/17] perf/x86/intel: Add EPT-Friendly PEBS for Ice Lake Server

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Message-ID: <YdSDEUJQgJQfZjWD@google.com>
Date:   Tue, 4 Jan 2022 17:25:37 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Like Xu <likexu@...cent.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v11 01/17] perf/x86/intel: Add EPT-Friendly PEBS for Ice
 Lake Server

On Fri, Dec 31, 2021, Like Xu wrote:
> On 31/12/2021 2:13 am, Sean Christopherson wrote:
> > On Fri, Dec 10, 2021, Like Xu wrote:
> > > The new hardware facility supporting guest PEBS is only available on
> > > Intel Ice Lake Server platforms for now. KVM will check this field
> > > through perf_get_x86_pmu_capability() instead of hard coding the cpu
> > > models in the KVM code. If it is supported, the guest PEBS capability
> > > will be exposed to the guest.
> > 
> > So what exactly is this new feature?  I've speed read the cover letter and a few
> > changelogs and didn't find anything that actually explained when this feature does.
> > 
> 
> Please check Intel SDM Vol3 18.9.5 for this "EPT-Friendly PEBS" feature.
> 
> I assume when an unfamiliar feature appears in the patch SUBJECT,
> the reviewer may search for the exact name in the specification.

C'mon, seriously?  How the blazes am I supposed to know that the feature name
is EPT-Friendly PEBS?  Or that it's even in the SDM (it's not in the year-old
version of the SDM I currently have open) versus one of the many ISE docs?

This is not hard.  Please spend the 30 seconds it takes to write a small blurb
so that reviewers don't have to spend 5+ minutes wondering WTF this does.

  Add support for EPT-Friendly PEBS, a new CPU feature that enlightens PEBS to
  translate guest linear address through EPT, and facilitates handling VM-Exits
  that occur when accessing PEBS records.  More information can be found in the
  <date> release of Intel's SDM, Volume 3, 18.9.5 "EPT-Friendly PEBS".