Date: Wed, 19 Feb 2014 20:48:32 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hp.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
        linux-arch@vger.kernel.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michel Lespinasse <walken@google.com>,
        Andi Kleen <andi@firstfloor.org>, Rik van Riel <riel@redhat.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
        George Spelvin <linux@horizon.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Daniel J Blueman <daniel@numascale.com>,
        Alexander Fyodorov <halcy@yandex.ru>,
        Aswin Chandramouleeswaran <aswin@hp.com>,
        Scott J Norton <scott.norton@hp.com>,
        Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@hp.com>
Subject: Re: [PATCH v4 0/3] qspinlock: Introducing a 4-byte queue spinlock
Message-ID: <20140219194832.GY14089@laptop.programming.kicks-ass.net>
References: <1392669684-4807-1-git-send-email-Waiman.Long@hp.com>
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 <5303B4C4.2040907@hp.com>
 <20140218212827.GR14089@laptop.programming.kicks-ass.net>
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 <20140219085107.GG27965@twins.programming.kicks-ass.net>
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On Wed, Feb 19, 2014 at 02:24:49PM -0500, Waiman Long wrote:
> On 02/19/2014 03:51 AM, Peter Zijlstra wrote:
> >On Tue, Feb 18, 2014 at 07:42:20PM -0500, Waiman Long wrote:
> >>On 02/18/2014 04:28 PM, Peter Zijlstra wrote:
> >>>On Tue, Feb 18, 2014 at 02:30:12PM -0500, Waiman Long wrote:
> >>>>I will start looking at how to make it work with paravirt. Hopefully, it
> >>>>won't take too long.
> >>>The cheap way out is to simply switch to the test-and-set spinlock on
> >>>whatever X86_FEATURE_ indicates a guest I suppose.
> >>I don't think there is X86_FEATURE flag that indicates running in a guest.
> >>In fact, a guest should never find out if it is running virtualized.
> >No it very much should; how else is paravirt ever going to work?
> 
> We do have a CONFIG_PARAVIRT macro that turns on or off PV support. The
> queue spinlock can be easily changed into an unfair lock which allows lock
> stealing. We could have a config option to make it unfair in the PARAVIRT
> environment, but I don't think Linus like the idea of an unfair lock.

No; a guest is very much aware of paravirt. See for example the
static_key_false(&paravirt_ticketlocks_enabled). It would be impossible
to set that branch if you never knew you were a guest.
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