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

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Message-ID: <87blvjarfo.fsf@vitty.brq.redhat.com>
Date:   Tue, 17 Sep 2019 17:11:23 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Jim Mattson <jmattson@...gle.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()

Jim Mattson <jmattson@...gle.com> writes:

> 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?

(I know not that much about scheduler so please bear with me)

Having a trustworthy scheduler not placing unrelated (not exposed as
sibling SMT threads to a guest or vCPUs from different guests) on
sibling SMT threads when it's not limited with affinity is definitely a
good thing. We, however, also need to know if vCPU pinning is planned
for the guest: like when QEMU vCPU threads are created they're not
pinned, however, libvirt pins them if needed before launching the
guest. So 'NoNonArchitecturalCoreSharing' can also be set in two cases:
- No vCPU pinning is planned but the scheduler is aware of the problem
(I'm not sure it is nowadays)
- The upper layer promises to do the right pinning.

This patch series, however, doesn't go that deep, it only covers the
simplest case: SMT is unavailable or forcefully disabled. I'll try to
learn more about scheduler though.

-- 
Vitaly