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SubjectRe: [PATCH] Re: UP APIC reenabling vs. cpu type detection ordering
On Thu, 8 Feb 2001, Mikael Pettersson wrote:

> No, poking into MSRs not explicitly defined on the current CPU is
> inherently unsafe. I have several x86 CPU data sheets here in front
> of me which say the same thing: "Don't write to undocumented MSRs."

Your point is right -- the problem are not undefined MSRs (that raise
#GP(0) on an access) but undocumented ones, sigh...

> You cannot assume that every single x86 out there stays clear of
> all Intel-defined MSRs. Intel has also expanded this set over time:
> older designs may not even have known about the APIC_BASE MSR.

Intel is actually sane -- you get #GP(0) for this MSR on P5. Others
might not and there are less cluefull vendors out there.

> 1. identify_cpu() (and more importantly get_cpu_vendor()) is called
> 2. include/asm-i386/processor.h #defines X86_VENDOR_INTEL as 0.
> 3. init/main.c calls time_init() before check_bugs() and thus
> 4. The cpu detection code rewrite in 2.4.0-test<something>

Yes, there are more problems as well. I'm working on it -- you may want
to look at the preliminary patch I sent here on Monday (strangely enough,
nobody out of linux-kernel seemed to be interested so far). Next version
should be available later this week -- the main problem with moving
identify_cpu() earlier are other cpu_data fields that get initialized
later, so more code needs to be actually rewritten.

> Ideally, identify_cpu() should be run before init_apic_mappings(),
> but my attempts to do so has so far had some weird side-effects
> (lost interrupts, incorrect bogomips, apparently stuck watchdog,
> and keyboard timeouts), so I won't touch that stuff.

You see. I'm going to move identify_cpu() very early anyway, but this
need a careful code review.


+ Maciej W. Rozycki, Technical University of Gdansk, Poland +
+ e-mail:, PGP key available +

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