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

From: Christopher Covington
Date: Wed Sep 24 2014 - 10:12:17 EST

Hi Nathan,

On 09/22/2014 08:28 PM, Nathan Lynch wrote:
> 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.

Just to clarify, were the performance improvements you measured from a
virtualized guest or native?

>> 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.

I count 18 dts* files that have "arm,armv7-timer", including platforms with
Krait, Exynos, and Tegra processors.


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