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

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Message-ID: <20140922223022.GO5182@n2100.arm.linux.org.uk>
Date:	Mon, 22 Sep 2014 23:30:23 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Will Deacon <will.deacon@....com>
Cc:	Nathan Lynch <nathan_lynch@...tor.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	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 Mon, Sep 22, 2014 at 04:39:19PM +0100, Will Deacon wrote:
> On Thu, Sep 18, 2014 at 03:59:32PM +0100, Nathan Lynch wrote:
> > This series contains the necessary changes to allow architected timer
> > access from user-space on 32-bit ARM.  This allows the VDSO to support
> > high resolution timestamps for clock_gettime and gettimeofday.  This
> > also merges substantially similar code from arm and arm64 into the
> > core arm_arch_timer driver.
> > 
> > The functional changes are:
> > - When available, CNTVCT is made readable by user space on arm, as it
> >   is on arm64.
> > - The clocksource name becomes "arch_mem_counter" if CP15 access to
> >   the counter is not available.
> > 
> > These changes have been carried as part of the ARM VDSO patch set over
> > the last several months, but I am splitting them out here as I assume
> > they should go through the clocksource maintainers.
> 
> For the series:
> 
>   Acked-by: Will Deacon <will.deacon@....com>
> 
> I'm not sure which tree the arch-timer stuff usually goes through, but
> the arm/arm64 bits look fine so I'm happy for them to merged together.

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.  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.

So, my view is that this VDSO will only be of very limited use for
32-bit ARM, and should not be exposed to userspace unless there is
a reason for it to be exposed (iow, the hardware necessary to support
it is present.)

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