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Subject: Re: [PATCH 1/3] cpu/SMT: create and export cpu_smt_possible()
To:     Jim Mattson <jmattson@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>
Cc:     kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
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        the arch/x86 maintainers <x86@kernel.org>,
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        Sean Christopherson <sean.j.christopherson@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Michael Kelley <mikelley@microsoft.com>,
        Roman Kagan <rkagan@virtuozzo.com>
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 <CALMp9eQP7Dup+mMuAiShNtH754R_Wwuvf63hezygh3TGR=a9rg@mail.gmail.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
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Date:   Tue, 17 Sep 2019 16:07:16 +0200
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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