svn commit: r314669 - head/sys/i386/conf

[John Baldwin]

On Saturday, March 04, 2017 10:52:46 AM Pedro Giffuni wrote:
> 
> On 03/04/17 10:32, Slawa Olhovchenkov wrote:
> > On Sat, Mar 04, 2017 at 03:04:17PM +0000, Pedro F. Giffuni wrote:
> >
> >> Author: pfg
> >> Date: Sat Mar  4 15:04:17 2017
> >> New Revision: 314669
> >> URL: https://svnweb.freebsd.org/changeset/base/314669
> >>
> >> Log:
> >>   Drop i486 from the default i386 GENERIC kernel configuration.
> >>
> >>   80486 production was stopped by Intel on September 2007. Dropping the 486
> >>   configuration option from the GENERIC kernel improves performance
> >>   slightly.
> >>
> >>   Removing I486_CPU is consistent at this time: we don't support any
> >>   processor without a FPU and the PC-98 arch, which frequently involved i486
> >>   CPUs, is also gone so we don't test such platforms anymore.
> >
> > What is realy mean?
> 
> This means we don't do work-arounds that would be required for raw 486.
> Instead we will use the 586 instructions by default.

This doesn't change that.  The kernel already has runtime tests in place
for new things on 486 and later via cpuid.

> > Some Via CPU is like i486 (by instruction set).
> >
> > CPU: VIA Ezra (800.04-MHz 686-class CPU)
> >   Origin="CentaurHauls"  Id=0x678  Family=0x6  Model=0x7  Stepping=8
> >   Features=0x803035<FPU,DE,TSC,MSR,MTRR,PGE,MMX>
> >   AMD Features=0x80000000<3DNow!>
> >
> 
> 486 never had MMX extensions.
> This is a 686, performance should improve ~4%.

How did you measure the improvement?  Keeping I486_CPU doesn't really
do anything except remove a some #ifdef'd conditionals in identcpu.c
and initcpu.c.  It doesn't affect whether we use the TSC, MMX, etc.  Those
are all runtime checks based the CPU feature flags from cpuid.

-- 
John Baldwin