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Subject: Re: do_gettimeofday vs. rdtsc in the scheduler
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Mikael Pettersson <mikpe@csd.uu.se>
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On Thu, 2002-09-19 at 12:20, Mikael Pettersson wrote:
>  > The local APIC timer is specified in the Intel Manual volume 3 for example.
>  > It's an optional feature (CPUID), but pretty much everyone has it.
> 
> Except that like everything else related to the local APIC, you're at
> the mercy of the competence (or lack thereof) of the BIOS implementors.
> - There are plenty of laptops whose CPUs have local APICs but whose
>   BIOSen go berserk if you enable it. There are also plenty of laptops

Frequently because we don't disable it again before any APM calls I
suspect. When a CPU goes into sleep mode you must disable PMC and local
apic timer interrupts.

>   that don't have one, since Intel removed it from many Mobile P6 CPUs.
> - There are even some desktop boards with BIOS problems, including Intel's
>   AL440LX on which Linux must stay away from the local APIC timer.
> 
> To assume the local APIC works on 686-class UP boxes is not realistic, alas.

Yep

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