linux-kernel - Re: disabling halt polling broken? (was Re: [PATCH 00/14] KVM: Halt-polling fixes, cleanups and a new stat)

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Message-ID: <CABgObfYtS6wiQe=BhF3t5usr7J6q4PWE4=rwZMMukfC9wT_6fA@mail.gmail.com>
Date:   Mon, 27 Sep 2021 19:24:52 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Jon Cargille <jcargill@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        KVM ARM <kvmarm@...ts.cs.columbia.edu>,
        "open list:MIPS" <linux-mips@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, kvm-ppc <kvm-ppc@...r.kernel.org>,
        "Kernel Mailing List, Linux" <linux-kernel@...r.kernel.org>,
        Jing Zhang <jingzhangos@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Janosch Frank <frankja@...ux.ibm.com>
Subject: Re: disabling halt polling broken? (was Re: [PATCH 00/14] KVM:
 Halt-polling fixes, cleanups and a new stat)

On Mon, Sep 27, 2021 at 5:17 PM Christian Borntraeger
<borntraeger@...ibm.com> wrote:
> > So I think there are two possibilities that makes sense:
> >
> > * track what is using KVM_CAP_HALT_POLL, and make writes to halt_poll_ns follow that
>
> what about using halt_poll_ns for those VMs that did not uses KVM_CAP_HALT_POLL and the private number for those that did.

Yes, that's what I meant.  David pointed out that doesn't allow you to
disable halt polling altogether, but for that you can always ask each
VM's userspace one by one, or just not use KVM_CAP_HALT_POLL. (Also, I
don't know about Google's usecase, but mine was actually more about
using KVM_CAP_HALT_POLL to *disable* halt polling on some VMs!).

Paolo