Re: [PATCH] x86,APIC: Detect lapic_is_integrated() once - use onand on.

From: Maciej W. Rozycki
Date: Fri May 22 2009 - 18:18:34 EST


On Fri, 22 May 2009, H. Peter Anvin wrote:

> > actually this change could be dangerous. I don't
> > remember if I saw mixed configuration at all but
> > I would not be that sure that we will never met it.

It is a matter of question whether this change gives any performance
benefit, but certainly it is not dangerous. What's the difference in code
generated?

Also APIC accesses cannot be cached, so access to the variable if hot in
the cache should be faster, but OTOH if cold, then main RAM access may
actually be slower as the APIC is quite closely coupled to the CPU. Have
any figures indicating performance change been obtained?

> That seems unlikely in the extreme. To the best of my knowledge, only
> 486s ever used the external APICs.

Several Pentium-based systems used external APICs (both ones built around
original P5 and ones using P54C chips), but mixed configurations were not
possible because of ICC bus/protocol incompatibility. First of all the
number of lines was different (5 vs 3)...

Maciej
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/