linux-kernel - Re: [PATCH v4 0/3] qspinlock: Introducing a 4-byte queue spinlock

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Message-ID: <20140219194832.GY14089@laptop.programming.kicks-ass.net>
Date:	Wed, 19 Feb 2014 20:48:32 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@...com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	George Spelvin <linux@...izon.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Daniel J Blueman <daniel@...ascale.com>,
	Alexander Fyodorov <halcy@...dex.ru>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>,
	Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@...com>
Subject: Re: [PATCH v4 0/3] qspinlock: Introducing a 4-byte queue spinlock

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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