linux-kernel - Re: Setting monotonic time?

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Date:   Wed, 03 Oct 2018 08:14:25 +0200
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Arnd Bergmann <arnd@...db.de>, avagin@...tuozzo.com,
        dima@...sta.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        0x7f454c46@...il.com, adrian@...as.de,
        Andy Lutomirski <luto@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        gorcunov@...nvz.org, "H. Peter Anvin" <hpa@...or.com>,
        Ingo Molnar <mingo@...hat.com>, Jeff Dike <jdike@...toit.com>,
        Oleg Nesterov <oleg@...hat.com>, xemul@...tuozzo.com,
        Shuah Khan <shuah@...nel.org>,
        containers@...ts.linux-foundation.org, criu@...nvz.org,
        Linux API <linux-api@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        linux-kselftest@...r.kernel.org
Subject: Re: Setting monotonic time?

Thomas Gleixner <tglx@...utronix.de> writes:

> On Wed, 3 Oct 2018, Eric W. Biederman wrote:
>> Direct access to hardware/drivers and not through an abstraction like
>> the vfs (an abstraction over block devices) can legitimately be handled
>> by hotplug events.  I unplug one keyboard I plug in another.
>> 
>> I don't know if the input layer is more of a general abstraction
>> or more of a hardware device.  I have not dug into it but my guess
>> is abstraction from what I have heard.
>> 
>> The scary difficulty here is if after restart input is reporting times
>> in CLOCK_MONOTONIC and the applications in the namespace are talking
>> about times in CLOCK_MONOTONIC_SYNC.  Then there is an issue.  As even
>> with a fixed offset the times don't match up.
>> 
>> So a time namespace absolutely needs to do is figure out how to deal
>> with all of the kernel interfaces reporting times and figure out how to
>> report them in the current time namespace.
>
> So you want to talk to Arnd who is leading the y2038 effort. He knowns how
> many and which interfaces are involved aside of the obvious core timer
> ones. It's quite an amount and the problem is that you really need to do
> that at the interface level, because many of those time stamps are taken in
> contexts which are completely oblivious of name spaces. Ditto for timeouts
> and similar things which are handed in through these interfaces.

Yep.  That sounds right.

Eric