linux-kernel - Re: [PATCH] rust: time: New module for timekeeping functions

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Message-ID: <65ab5cb1-2dbe-e853-48ea-d1df87706cf0@redhat.com>
Date:   Tue, 21 Feb 2023 09:13:24 -0800
From:   Josh Stone <jistone@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Asahi Lina <lina@...hilina.net>,
        Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        John Stultz <jstultz@...gle.com>,
        Stephen Boyd <sboyd@...nel.org>
Cc:     linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
        asahi@...ts.linux.dev
Subject: Re: [PATCH] rust: time: New module for timekeeping functions

On 2/21/23 4:32 AM, Thomas Gleixner wrote:
> Now the problem is that 'Instant' in it's specification is bound to
> CLOCK_MONOTONIC and there is no way to express CLOCK_BOOTTIME, but
> that's a shortcoming of the spec which ignores CLOCK_BOOTTIME
> completely. IOW, that's also a problem for user space.

That's not exactly *specified* -- it's meant to be opaque time. It is
documented that this currently uses clock_gettime monotonic on unix
targets, but "Disclaimer: These system calls might change over time."
CLOCK_MONOTONIC isn't even consistent across unix targets whether that
counts suspended time. It's been debated if we should switch to
CLOCK_BOOTTIME on Linux, but for now we're sticking to monotonic:

https://github.com/rust-lang/rust/pull/88714

But as others mentioned, this is in the std crate which isn't used by
the kernel. You could absolutely define your own with more specificity,
like kernel::time::BootTime, and copy an API similar to Instant's.