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

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Message-ID: <4704aac9-cac2-f04d-8344-6642432c31e2@redhat.com>
Date:   Tue, 17 Sep 2019 16:07:16 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>,
        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>,
        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 16/09/19 19:16, Jim Mattson 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?

Yes, however that scheduler API could also rely on something like
cpu_smt_possible(), at least in the case where core scheduling is not
active, so this is still a step in the right direction.

Paolo