linux-kernel - Re: [PATCH] kvm: add capability for halt polling

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Message-ID: <CANxmayh3A1CaLqKdf=GknOmOQ-ww0npg9ikUTGRyDbbRge+yHQ@mail.gmail.com>
Date:   Mon, 20 Apr 2020 14:20:34 -0700
From:   Jon Cargille <jcargill@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        David Matlack <dmatlack@...gle.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm: add capability for halt polling

On Mon, Apr 20, 2020 at 2:10 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 20/04/20 20:47, Jon Cargille wrote:
> >> Is it safe to allow any value from userspace here or would it maybe make
> >> sense to only allow [0, global halt_poll_ns]?
> > I believe that any value is safe; a very large value effectively disables
> > halt-polling, which is equivalent to setting a value of zero to explicitly
> > disable it, which is legal.
>
> Doesn't a large value make KVM poll all the time?  But you could do that
> just by running "for (;;)" so there's no reason to limit the parameter.

Yes, I mis-spoke; apologies. A large number will cause KVM to poll for a
long time; as long as the thread can be preempted, we don't see any
problem with that.

Thanks,

Jon

>
> Paolo
>