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Date:   Mon, 22 Oct 2018 19:31:44 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Juergen Gross <jgross@suse.com>, Yi Sun <yi.y.sun@linux.intel.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org, tglx@linutronix.de,
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        isaku.yamahata@intel.com, michael.h.kelley@microsoft.com,
        tianyu.lan@microsoft.com, "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "mingo@redhat.com" <mingo@redhat.com>,
        Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH v1 2/2] x86/hyperv: make HvNotifyLongSpinWait hypercall
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On Mon, Oct 22, 2018 at 01:27:27PM -0400, Waiman Long wrote:

> >> I agree with Juergen on that. I would suggest rename the
> >> vcpu_is_preempted hook into a more generic vcpu_stop_spinning, perhaps,
> >> so different hypervisors can act on the information accordingly. Adding
> >> an extra parameter is fine.
> > No; no extra parameters. vcpu_is_preempted() is a simple and intuitive
> > interface. Why would we want to make it complicated?
> 
> Hyperv seems to do it in a somewhat different way by looking at the spin
> counter and decide if it should continue. I don't know why they do that
> and what advantage it has.
> 
> The current patch is definitely not OK. A revised patch that makes use
> of an existing paravirt hook will be more acceptable. Again, I would
> like to see some performance figure and why they do it this way to see
> if it is worthwhile to change the existing interface.

Note that there are vcpu_is_preempted() users that are not in a
spin-loop.