linux-kernel - Re: [GIT PULL] timer fixes

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Date:   Tue, 17 Dec 2019 12:43:46 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [GIT PULL] timer fixes

On Tue, Dec 17, 2019 at 12:35 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> Pray.. the TSC MSR is still writable from SMM, so BIOS monkeys could
> still do what they've been doing for decades.

Sure. And the HPET is unreliable, so the checking causes issues.

Which one should we worry about?

> Also, what consititutes a 'modern' CPU?

I think anything that has TSC_STABLE set should likely be considered
more reliable than HPET.

Or whatever the bit is called. The "doesn't stop in idle" thing.

> > The HPET seems to get disabled on all the modern platforms, why do we
> > even have it enabled by default?
>
> These new ones yeah, cuz they wrecked HPET in PC10 :/

That's my point. HPET isn't _used_ by Windows, so it gets no testing,
so it will continue to have bugs.

At least the TSC is used.

I do agree about the whole SMM issue, but it seems less of a pain than
the HPET issue.

             Linus