linux-kernel - Re: [PATCH v2 0/3] arm_arch_timer: VDSO preparation, code consolidation

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Date:	Wed, 24 Sep 2014 15:52:57 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Will Deacon <Will.Deacon@....com>,
	Nathan Lynch <nathan_lynch@...tor.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Christopher Covington <cov@...eaurora.org>,
	Doug Anderson <dianders@...omium.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Mark Rutland <Mark.Rutland@....com>,
	Sonny Rao <sonnyrao@...omium.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/3] arm_arch_timer: VDSO preparation, code
	consolidation

On Wed, Sep 24, 2014 at 03:45:41PM +0100, Catalin Marinas wrote:
> On Mon, Sep 22, 2014 at 11:30:23PM +0100, Russell King - ARM Linux wrote:
> > I raised a while back with Will whether there's much point to having
> > this on ARM.  While it's useful for virtualisation, the majority of
> > 32-bit ARM doesn't run virtualised.
> 
> This has nothing to do with virtualisation. The main reason we use
> CNTVCT is to not require kernel binary differences when running the OS
> as host or guest. But it does _not_ mean that it is only used when
> running as a guest.
> 
> > So there's little point in having the VDSO on the majority of
> > platforms - it will just add additional unnecessary cycles slowing
> > down the system calls that the VDSO is designed to try to speed up.
> 
> A good reason for VDSO is to avoid a system call for gettimeofday when
> you can read the clocks source from user space. That's a significant
> improvement on CPUs like A7, A15.

I'm *not* arguing against having a VDSO to speed up that crap.  What
I'm trying to get to the bottom of - something which has been totally
lost sight of - is what the friggin effect of this stuff is on CPUs
*without* the architected timer.

Until I get an answer to what the measured effect is, I'm saying no to
VDSO on ARM, because - as seems to be the norm - the evaluation job is
only half done.

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