From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1751465AbdIEIKR (ORCPT <rfc822;w@1wt.eu>);
        Tue, 5 Sep 2017 04:10:17 -0400
Received: from merlin.infradead.org ([205.233.59.134]:49790 "EHLO
        merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1750929AbdIEIKN (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Tue, 5 Sep 2017 04:10:13 -0400
Date: Tue, 5 Sep 2017 10:10:01 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Juergen Gross <jgross@suse.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>,
        Oscar Salvador <osalvador@suse.de>, Ingo Molnar <mingo@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>, "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, linux-doc@vger.kernel.org,
        Waiman Long <longman@redhat.com>
Subject: Re: [PATCH resend] x86,kvm: Add a kernel parameter to disable PV
 spinlock
Message-ID: <20170905081001.hn2276qrhfyqpjdi@hirez.programming.kicks-ass.net>
References: <20170904142836.15446-1-osalvador@suse.de>
 <20170904144011.gp7hpis6usjehbuf@hirez.programming.kicks-ass.net>
 <20170904222157.GD17982@linux-80c1.suse>
 <0869e8a5-4abd-8f7f-0135-aab3e72e2d01@suse.com>
 <20170905065837.rs767a4os2aumg7h@hirez.programming.kicks-ass.net>
 <924fec17-548a-083d-edce-7adcb662c513@suse.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <924fec17-548a-083d-edce-7adcb662c513@suse.com>
User-Agent: NeoMutt/20170609 (1.8.3)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

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.