Re: [patch] i386: avoid expensive ppro ordering workaround for default 686 kernels

[Nick Piggin <npiggin@suse.de>]

On Wed, Jan 02, 2008 at 01:44:33PM +0000, Alan Cox wrote:
> > Take a different approach: after this patch, we just disable all but one CPU on those
> > systems, and print a warning. Also printed is a suggestion for a new CONFIG option that
> > can be enabled for the previous behaviour.
> 
> How does that help. The processor isn't the only bus master.

Hmm, I don't understand what you mean. Obviously other busmasters aren't
participating in any locking or smp_*mb() ordering protocols.

The non-smp_-prefixed barriers are retained.

 
> Maybe this works as a SuSE specific convenience solution aligned to
> your particular build pattern but it isn't the right solution for
> upstream IMHO. 

Actually it is nothing to do with SUSE but I was using SLES as a counterexample
when you said nobody would care about M686 with SMP builds. The reason for the
patch is because I noticed the suboptimal configuration (because Andrew flagged
my patch).
 

> We should either
> 
> - re-order the assumed processor generations supported to put VIA C3/C5
> above Preventium Pro

Adrian Bunk's patch to make each CPU type explicitly selectable IMO is the
best way to do this.


> - fix the gcc or gcc settings not to generate invalid cmov instructions
> on 686. cmov is slower on all the modern processors anyway.

Not for unpredictable branches, but I agree gcc seems to use it too often
(in my version, it seems to even use it for likely/unlikely branches :( ).

 
> And you change the assumption that 586 < 686 < PPro < PII < PIII ...
> 
> to 586 < 686 < PPro < C3 < PII < ...
> 
> then selecting VIA C3 support will get you a kernel with the properties
> all the distribution vendors want for their higher end mainstream kernel -
> "runs on modern systems".

I don't agree. If we support those options, we should support them properly.
And if you build an SMP kernel for a 586 for example, you should not get lumped
with those pentiumpro workarounds.

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