linux-kernel - Re: [PATCH] x86/tsc: Add debugfs entry to mark TSC as unstable after boot

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Message-ID: <EA2BAF2F-3F8E-4F81-B71C-7B97677216C9@zytor.com>
Date: Mon, 17 Mar 2025 11:42:30 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: "Guilherme G. Piccoli" <gpiccoli@...lia.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
CC: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, kernel@...ccoli.net,
        kernel-dev@...lia.com
Subject: Re: [PATCH] x86/tsc: Add debugfs entry to mark TSC as unstable after boot

On March 17, 2025 7:35:45 AM PDT, "Guilherme G. Piccoli" <gpiccoli@...lia.com> wrote:
>On 26/02/2025 10:27, Guilherme G. Piccoli wrote:
>> Right now, we can force the TSC to be marked as unstable through
>> boot parameter. There are debug / test cases though in which would
>> be preferable to simulate the clocksource watchdog behavior, i.e.,
>> marking TSC as unstable during the system run. Some paths might
>> change, for example: the tracing clock is auto switched to global
>> if TSC is marked as unstable on boot, but it could remain local if
>> TSC gets marked as unstable after tracing initialization.
>> 
>> Hence, the proposal here is to have a simple debugfs file that
>> gets TSC marked as unstable when written.
>> 
>> Signed-off-by: Guilherme G. Piccoli <gpiccoli@...lia.com>
>> ---
>>  arch/x86/kernel/tsc.c | 22 ++++++++++++++++++++++
>>  1 file changed, 22 insertions(+)
>
>Hi folks, gentle ping about this one - any suggestions?
>Cheers,
>
>
>Guilherme

To be honest I don't think this belongs in debugfs; rather it belongs in sysfs.

Debugfs should not be necessarily in serious production systems – it is way too large of an attack surface, which is a very good reason why it is its own filesystem – but if this is a real issue on hardware then it may be needed.

   -hpa