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From: David Matlack <dmatlack@google.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Venkatesh Srinivas <venkateshs@chromium.org>,
	kvm list <kvm@vger.kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH] kvm: Cap halt polling at kvm->max_halt_poll_ns
Date: Thu, 6 May 2021 09:30:14 -0700	[thread overview]
Message-ID: <CALzav=e+6mtPUHTHFLbw1Q=1kgstPbAp=2mg9mi+_fc+iWELGA@mail.gmail.com> (raw)
In-Reply-To: <YJQVj3GaVp9tvWog@google.com>

On Thu, May 6, 2021 at 9:13 AM Sean Christopherson <seanjc@google.com> wrote:
>
> Prefer capitalizing KVM in the shortlog, if only because I'm lazy with grep :-)
>
> On Thu, May 06, 2021, Venkatesh Srinivas wrote:
> > From: David Matlack <dmatlack@google.com>
> >
> > When growing halt-polling, there is no check that the poll time exceeds
> > the per-VM limit. It's possible for vcpu->halt_poll_ns to grow past
> > kvm->max_halt_poll_ns and stay there until a halt which takes longer
> > than kvm->halt_poll_ns.
> >
>
> Fixes: acd05785e48c ("kvm: add capability for halt polling")
>
> and probably Cc: stable@ too.
>
>
> > Signed-off-by: David Matlack <dmatlack@google.com>
> > Signed-off-by: Venkatesh Srinivas <venkateshs@chromium.org>
> > ---
> >  virt/kvm/kvm_main.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 2799c6660cce..120817c5f271 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -2893,8 +2893,8 @@ static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
> >       if (val < grow_start)
> >               val = grow_start;
> >
> > -     if (val > halt_poll_ns)
> > -             val = halt_poll_ns;
> > +     if (val > vcpu->kvm->max_halt_poll_ns)
> > +             val = vcpu->kvm->max_halt_poll_ns;
>
> Hmm, I would argue that the introduction of the capability broke halt_poll_ns.
> The halt_poll_ns module param is writable after KVM is loaded.  Prior to the
> capability, that meant the admin could adjust the param on the fly and all vCPUs
> would honor the new value as it was changed.
>
> By snapshotting the module param at VM creation, those semantics were lost.
> That's not necessarily wrong/bad, but I don't see anything in the changelog for
> the capability that suggests killing the old behavior was intentional/desirable.

api.rst does say the capability overrides halt_poll_ns. But I could
see value in changing the semantics to something like:

- halt_poll_ns sets machine-wide maximum halt poll time.
- kvm->max_halt_poll_ns sets VM-wide maximum halt poll time.
- A vCPU will poll for at most min(halt_poll_ns,
kvm->max_halt_poll_ns) (aside from an in-progress poll when either
parameter is changed).

On a related note, the capability and these subtle details should be
documented in Documentation/virtual/kvm/halt-polling.txt.


>
> >
> >       vcpu->halt_poll_ns = val;
> >  out:
> > --
> > 2.31.1.607.g51e8a6a459-goog
> >

  reply	other threads:[~2021-05-06 16:30 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-06 15:24 Venkatesh Srinivas
2021-05-06 16:13 ` Sean Christopherson
2021-05-06 16:30   ` David Matlack [this message]
2021-05-06 17:41     ` Sean Christopherson

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