linux-kernel - Re: [RFC PATCH V2 2/9] perf: Extend ABI to support post-processing monotonic raw conversion

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Y+tl1ZJiWuMeKCnB@hirez.programming.kicks-ass.net>
Date:   Tue, 14 Feb 2023 11:43:33 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     John Stultz <jstultz@...gle.com>
Cc:     "Liang, Kan" <kan.liang@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
        linux-kernel@...r.kernel.org, sboyd@...nel.org, eranian@...gle.com,
        namhyung@...nel.org, ak@...ux.intel.com, adrian.hunter@...el.com
Subject: Re: [RFC PATCH V2 2/9] perf: Extend ABI to support post-processing
 monotonic raw conversion

On Mon, Feb 13, 2023 at 02:22:39PM -0800, John Stultz wrote:
> The interpoloation is pretty easy to do:
> 
> do {
>     start= readtsc();
>     clock_gett(CLOCK_MONOTONIC_RAW, &ts);
>     end = readtsc();
>     delta = end-start;
> } while (delta  > THRESHOLD)   // make sure the reads were not preempted
> mid = start + (delta +(delta/2))/2; //round-closest
> 
> and be able to get you a fairly close matching of TSC to
> CLOCK_MONOTONIC_RAW value.
> 
> Once you have that mapping you can take a few samples and establish
> the linear function.

Right, this is how we do the TSC calibration in the first place, and if
NTP can achieve high correctness over a network, then surely we can do
better locally.

That is, this scheme should work for all CLOCKs, not only MONOTONIC_RAW.