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

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Date:	Wed, 24 Sep 2014 16:08:07 +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 04:04:09PM +0100, Catalin Marinas wrote:
> On Wed, Sep 24, 2014 at 03:52:57PM +0100, Russell King - ARM Linux wrote:
> > 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.
> 
> I agree.
> 
> If there is an overhead (possibly), I think it can be solved in software
> maybe by having two VDSO images, one with gettimeofday and one without.
> If it's only gettimeofday in VDSO (and signal return still via the
> vectors page), we could just avoid inserting it into the user address
> space when arch timers aren't present.

The signal handling is no longer in the vectors page (it hasn't been for
over a year now), it is in a separate page which is mapped randomly.
These VDSO patches change it to place it along side the VDSO pages.

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