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

From: Nathan Lynch
Date: Mon Sep 22 2014 - 20:28:15 EST

On 09/22/2014 05:30 PM, Russell King - ARM Linux wrote:
> 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@xxxxxxx>
>> 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.

Hmm, this patch set is merely exposing the hardware counter when it is
present for the VDSO's use; I take it you have no objection to that?

While the 32-bit ARM VDSO I've posted (in a different thread) exploits a
facility that is required by the virtualization option in the
architecture, its utility is not limited to guest operating systems.

> 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.)

My thinking is that it should prove useful in a growing subset of v7
CPUs. It is useful today on Cortex-A15 and -A7, and I believe -A12 and
-A17 implement the generic timer facility as well.

Now if you're saying that we shouldn't slow down gettimeofday on systems
which lack a hardware counter that can be safely exposed to userspace, I
can work with that.

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