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Re: [PATCH 6/6] aarch64: Add hp-timing.h
- From: Marcus Shawcroft <marcus dot shawcroft at gmail dot com>
- To: Andreas Schwab <schwab at suse dot de>
- Cc: pinskia at gmail dot com, Richard Henderson <rth at twiddle dot net>, "libc-alpha at sourceware dot org" <libc-alpha at sourceware dot org>, "azanella at linux dot vnet dot ibm dot com" <azanella at linux dot vnet dot ibm dot com>, "davem at davemloft dot net" <davem at davemloft dot net>, "marcus dot shawcroft at arm dot com" <marcus dot shawcroft at arm dot com>
- Date: Mon, 21 Jul 2014 19:31:31 +0100
- Subject: Re: [PATCH 6/6] aarch64: Add hp-timing.h
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- References: <1403735086-21797-1-git-send-email-rth at twiddle dot net> <1403735086-21797-7-git-send-email-rth at twiddle dot net> <mvmd2czlz0k dot fsf at hawking dot suse dot de> <812AFAEE-C8E5-4583-8647-7E5ABE02F95C at gmail dot com> <mvm4myanco3 dot fsf at hawking dot suse dot de>
On 21 July 2014 13:01, Andreas Schwab <schwab@suse.de> wrote:
> pinskia@gmail.com writes:
>
>>> On Jul 21, 2014, at 4:41 AM, Andreas Schwab <schwab@suse.de> wrote:
>>>
>>> Richard Henderson <rth@twiddle.net> writes:
>>>
>>>> +/* Sync the instruction stream, and read from the virtual cycle counter. */
>>>> +#define HP_TIMING_NOW(Var) \
>>>> + __asm__ __volatile__ ("isb; mrs %0, cntvct_el0" : "=r" (Var))
>>>
>>> According to https://bugs.launchpad.net/bugs/1344320 the generic timers
>>> are not part of the kernel-to-userspace contract.
>>
>>
>> I think this is bogus for the kernel folks not allow a high precision timer in user space. Timers like this are needed for micro-benchmarking compiler changes along with other libc changes.
>
> According to the qemu bug the timers are optional.
>
The generic timers are indeed optional, therefore we should not assume
they are always present. I don't want this to ship in 2.20 as is, so
will back out the patch in the morning pending a better solution.
/Marcus