linux-kernel - Re: [Y2038][time namespaces] Question regarding CLOCK_REALTIME support plans in Linux time namespaces

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Date:   Fri, 30 Oct 2020 16:43:37 +0100
From:   Lukasz Majewski <lukma@...x.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andrei Vagin <avagin@...il.com>, Dmitry Safonov <dima@...sta.com>,
        GNU C Library <libc-alpha@...rceware.org>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org
Subject: Re: [Y2038][time namespaces] Question regarding CLOCK_REALTIME
 support plans in Linux time namespaces

Hi Thomas,

> Lukasz,
> 
> On Fri, Oct 30 2020 at 11:02, Lukasz Majewski wrote:
> > I do have a question regarding the Linux time namespaces in respect
> > of adding support for virtualizing the CLOCK_REALTIME.
> >
> > According to patch description [1] and time_namespaces documentation
> > [2] the CLOCK_REALTIME is not supported (for now?) to avoid
> > complexity and overhead in the kernel.
> >
> > Is there any plan to add support for it in a near future?  
> 
> Not really. Just having an offset on clock realtime would be incorrect
> in a number of ways. Doing it correct is a massive trainwreck.
> 
> For a debug aid, which is what you are looking for, the correctness
> would not really matter, but providing that is a really slippery
> slope.
> 
> If at all we could hide it under a debug option which depends on
> CONFIG_BROKEN and emitting a big fat warning in dmesg with a clear
> statement that it _is_ broken, stays so forever and any attempt to
> "fix" it results in a permanent ban from all kernel lists.
> 
> Preferrably we don't go there.

I see. Thanks for the explanation.

Now, I do use QEMU to emulate ARM 32 bit system with recent kernel
(5.1+). It works. 

Another option would be to give a shoot to QEMU with the "user mode" to
run cross-compiled tests (with using a cross-compiled glibc in earlier
CI stage). The problem with above is the reliance on QEMU emulation of
ARM syscalls (and if 64 bit time supporting syscalls - i.e.
clock_settime64 - are available).

> 
> Thanks,
> 
>         tglx
> 
> 




Best regards,

Lukasz Majewski

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