linux-kernel - Re: [PATCH] x86/ioapic: don't use unstable TSC to detect timer IRQ

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Message-ID: <20180320090931.GQ4043@hirez.programming.kicks-ass.net>
Date:   Tue, 20 Mar 2018 10:09:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Liu, Changcheng" <changcheng.liu@...el.com>
Cc:     tglx@...utronix.de, hpa@...or.com, douly.fnst@...fujitsu.com,
        linux-kernel@...r.kernel.org,
        "Stanton, Kevin B" <kevin.b.stanton@...el.com>
Subject: Re: [PATCH] x86/ioapic: don't use unstable TSC to detect timer IRQ

On Tue, Mar 20, 2018 at 04:58:35PM +0800, Liu, Changcheng wrote:
> On 09:49 Tue 20 Mar, Peter Zijlstra wrote:
> > On Tue, Mar 20, 2018 at 04:42:55PM +0800, Liu, Changcheng wrote:
> > > In rare case, the TSC is every unstable or can't sync with
> > > real time hardware clock. 
> > 
> > However did you manage that? Please provide _FAR_ more details.

> [Changcheng] TSC is simulated and HPET is hardware implemented.
> TSC can't sync with HPET. When running linux, the TSC grows too
> fast and HPET can't trigger periodic timer interrupt in time which
> is used to update jiffies.

How is that not utterly broken, and how is that even allowed behaviour
as per the SDM ?

If the TSC is so utterly broken as to not function according to spec,
you should not advertise the TSC, at which point you'll find that it is
_very_ hard to run modern linux on x86 without TSC.