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: <YVHgKWiU9WWL9ACg@google.com>
Date:   Mon, 27 Sep 2021 15:15:53 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Christian Borntraeger <borntraeger@...ibm.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@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, 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, Paolo Bonzini wrote:
> On 27/09/21 16:59, Sean Christopherson wrote:
> > > commit acd05785e48c01edb2c4f4d014d28478b5f19fb5
> > > Author:     David Matlack<dmatlack@...gle.com>
> > > AuthorDate: Fri Apr 17 15:14:46 2020 -0700
> > > Commit:     Paolo Bonzini<pbonzini@...hat.com>
> > > CommitDate: Fri Apr 24 12:53:17 2020 -0400
> > > 
> > >      kvm: add capability for halt polling
> > > 
> > > broke the possibility for an admin to disable halt polling for already running KVM guests.
> > > In past times doing
> > > echo 0 > /sys/module/kvm/parameters/halt_poll_ns
> > > 
> > > stopped polling system wide.
> > > Now all KVM guests will use the halt_poll_ns value that was active during
> > > startup - even those that do not use KVM_CAP_HALT_POLL.
> > > 
> > > I guess this was not intended?
> 
> No, but...
> 
> > I would go so far as to say that halt_poll_ns should be a hard limit on
> > the capability
> 
> ... this would not be a good idea I think.  Anything that wants to do a lot
> of polling can just do "for (;;)".

Hmm, true, there is no danger to the system in having the capability override the
module param.

> 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

I think this option makes more sense, making halt_poll_ns read-only is basically
forcing users to switch to KVM_CAP_HALT_POLL.

> * just make halt_poll_ns read-only.
> 
> Paolo
>