linux-kernel - Re: [PATCH 1/3] cpu/SMT: create and export cpu_smt

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Message-ID: <CALMp9eQP7Dup+mMuAiShNtH754R_Wwuvf63hezygh3TGR=a9rg@mail.gmail.com>
Date:   Mon, 16 Sep 2019 10:16:17 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-hyperv@...r.kernel.org,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Roman Kagan <rkagan@...tuozzo.com>
Subject: Re: [PATCH 1/3] cpu/SMT: create and export cpu_smt_possible()

On Mon, Sep 16, 2019 at 9:23 AM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
> KVM needs to know if SMT is theoretically possible, this means it is
> supported and not forcefully disabled ('nosmt=force'). Create and
> export cpu_smt_possible() answering this question.

It seems to me that KVM really just wants to know if the scheduler can
be trusted to avoid violating the invariant expressed by the Hyper-V
enlightenment, NoNonArchitecturalCoreSharing. It is possible to do
that even when SMT is enabled, if the scheduler is core-aware.
Wouldn't it be better to implement a scheduler API that told you
exactly what you wanted to know, rather than trying to infer the
answer from various breadcrumbs?