linux-kernel - Re: [PATCH resend] x86,kvm: Add a kernel parameter to disable PV spinlock

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Message-ID: <20170904144011.gp7hpis6usjehbuf@hirez.programming.kicks-ass.net>
Date:   Mon, 4 Sep 2017 16:40:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, Waiman Long <longman@...hat.com>
Subject: Re: [PATCH resend] x86,kvm: Add a kernel parameter to disable PV
 spinlock

On Mon, Sep 04, 2017 at 04:28:36PM +0200, Oscar Salvador wrote:
> This is just a resend of Waiman Long's patch.
> I could not find why it was not merged to upstream, so I thought
> to give it another chance.
> What follows is what Waiman Long wrote.
> 
> Xen has an kernel command line argument "xen_nopvspin" to disable
> paravirtual spinlocks. This patch adds a similar "kvm_nopvspin"
> argument to disable paravirtual spinlocks for KVM. This can be useful
> for testing as well as allowing administrators to choose unfair lock
> for their KVM guests if they want to.

For testing its trivial to hack your kernel and I don't feel this is
something an Admin can make reasonable decisions about.

So why? In general less knobs is better.