linux-kernel - Re: [PATCH v2 08/13] x86/vdso: Fix gettimeofday masking

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Message-ID: <87y1l4dsqr.ffs@tglx>
Date:   Thu, 01 Jun 2023 00:46:20 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>, bigeasy@...utronix.de
Cc:     mark.rutland@....com, maz@...nel.org, catalin.marinas@....com,
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Subject: Re: [PATCH v2 08/13] x86/vdso: Fix gettimeofday masking

On Wed, May 31 2023 at 17:27, Thomas Gleixner wrote:
> On Fri, May 19 2023 at 12:21, Peter Zijlstra wrote:
>> to take wrapping into account, but per all the above, we don't
>> actually wrap on u64 anymore.
>
> Indeed. The rationale was that you need ~146 years uptime with a 4GHz
> TSC or ~584 years with 1GHz to actually reach the wrap around point.
>
> Though I can see your point to make sure that silly BIOSes or VMMs
> cannot cause havoc by accident or malice.
>
> Did anyone ever validate that wrap around on TSC including TSC deadline
> timer works correctly?
>
> I have faint memories of TSC_ADJUST, which I prefer not to bring back to
> main memory :)

It seems my fears have been unjustified.

At least a quick test which sets the TSC to ~ -8min @2.1Ghz the machine
seems to survive without the colourful explosions I expected due to my
early exposure to TSC_ADJUST and TSC_DEADLINE_TIMER :)

Thanks,

        tglx