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

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Message-ID: <20170905081001.hn2276qrhfyqpjdi@hirez.programming.kicks-ass.net>
Date:   Tue, 5 Sep 2017 10:10:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juergen Gross <jgross@...e.com>
Cc:     Davidlohr Bueso <dave@...olabs.net>,
        Oscar Salvador <osalvador@...e.de>,
        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 Tue, Sep 05, 2017 at 09:35:40AM +0200, Juergen Gross wrote:
> > So the problem with qspinlock is that it will revert to a classic
> > test-and-set spinlock if you don't do paravirt but are running a HV.
> 
> In the Xen case we just use the bare metal settings when xen_nopvspin
> has been specified. So paravirt, but without modifying any pv_lock_ops
> functions.

See arch/x86/include/asm/qspinlock.h:virt_spin_lock(). Unless you clear
X86_FEATURE_HYPERVISOR you get a test-and-set spinlock.

And as the comment there says, this is a fallback for !paravirt enabled
hypervisors to avoid the worst of the lock holder preemption crud.

But this very much does not deal with the 1:1 case nicely.