linux-kernel - Re: Fwd: [PATCH v4.15.7 1/1] on Intel, VDSO should handle CLOCK_MONOTONIC_RAW and export 'tsc

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Message-ID: <8tazgkc7.fsf@gmail.com>
Date:   Sun, 11 Mar 2018 07:08:56 +0000
From:   Jason Vas Dias <jason.vas.dias@...il.com>
To:     x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        andi <andi@...stfloor.org>, Peter Zijlstra <peterz@...radead.org>
Subject: Re: Fwd: [PATCH v4.15.7 1/1] on Intel, VDSO should handle CLOCK_MONOTONIC_RAW and export 'tsc_calibration' pointer

Hi Thomas -

Thanks very much for your help & guidance in previous mail:

RE: On 08/03/2018, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> The right way to do that is to put the raw conversion values and the raw
> seconds base value into the vdso data and implement the counterpart of
> getrawmonotonic64(). And if that is done, then it can be done for _ALL_
> clocksources which support VDSO access and not just for the TSC.
>

I have done this now with a new patch, sent in mail with subject :

'[PATCH v4.16-rc4 1/1] x86/vdso: on Intel, VDSO should handle CLOCK_MONOTONIC_RAW' 

which should address all the concerns you raise.

> I already  know how that works, really.

I never doubted or meant to impugn that !

I am beginning to know a little how that works also, thanks in great
part to your help last week - thanks for your patience.

I was impatient last week to get access to low latency timers for a work
project, and was trying to read the unadjusted clock .

> instead of making completely false claims about the correctness of the kernel
> timekeeping infrastructure.

I really didn't mean to make any such claims - I'm sorry if I did .  I was just trying
to say that by the time the results of clock_gettime(CLOCK_MONOTONIC_RAW,&ts) were
available to the caller they were not of much use because of the
latencies often dwarfing the time differences .

Anyway, I hope sometime you will consider putting such a patch in the
kernel.

I have developed a verson for ARM also, but that depends on making
CNTPCT + CNTFRQ registers readable in user-space, which is not meant
to be secure and is not normally done , but does work - but it is
against the Texas Instruments (ti-linux) kernel and can be enabled
with a new KConfig option, and brings latencies down from > 300ns
to < 20ns . Maybe I should post that also to kernel.org, or to
ti.com ?

I have a separate patch for the vdso_tsc_calibration export of the
tsc_khz and calibration which no longer returns pointers into the VDSO -
I can post this as a patch if you like.

Thanks & Best Regards,
Jason Vas Dias <jason.vas.dias@...il.com>

View attachment "vdso_monotonic_raw-v4.16-rc4.patch" of type "text/x-patch" (5066 bytes)