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

From: Russell King - ARM Linux
Date: Wed Sep 24 2014 - 11:08:37 EST

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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