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

[Nathan Lynch]

On 09/24/2014 09:12 AM, Christopher Covington wrote:
> 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?

Yeah I should have been explicit about this.  My tests and measurements
(and all test results I've received from others, I believe) have been on
native/host kernels, not guests.


>>> 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.

Yup.

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