Re: [RFC -tip] x86,apic -- reduce disable_apic usage

["Maciej W. Rozycki" <macro@linux-mips.org>]

On Sun, 5 Jul 2009, H. Peter Anvin wrote:

> >  How do you set cpu_has_apic for systems with discrete local APICs?  The 
> > CPUID flag is not set in this case.
> > 
> 
> Well, should it be?  We do set flags when they're appropriate to us, and
> if the semantics are such as that is inappropriate we can set a custom bit.

 Hmm, that might simplify things here and there and the less special cases 
in code -- and thus effort needed -- for the discrete APIC, the better.  
I think there is no reason why it couldn't be done -- all the places which 
need version-specific APIC features have to check the LVR register anyway.  
And the availability of the APICBASE MSR has to be validated separately 
too as it comes with P6+ only.

 The only place which could care I believe is code to set X86_FEATURE_11AP 
-- this should obviously be disabled for the discrete APIC as it is now, 
as the chip does not suffer from the erratum and the workaround is costly 
performance-wise.  That piece of code would have to be checked -- I don't 
know what the order of setting of these bits would be and thus if one 
could affect the other.  The dependency would better be well documented 
then too -- my observation is the knowledge about the APIC subsystem among 
people typically only covers a narrow subset of implementations.

  Maciej