Kernel Optimizations for Processors

[admin at forkthepenguin.com]

On Mon, 29 Dec 2003, John Baldwin wrote:

>
> On 29-Dec-2003 admin at forkthepenguin.com wrote:
> >> > # NO_F00F_HACK disables the hack that prevents Pentiums (and ONLY
> >> > # Pentiums) from locking up when a LOCK CMPXCHG8B instruction is
> >> > # executed.  This option is only needed if I586_CPU is also defined,
> >> > # and should be included for any non-Pentium CPU that defines it.
> >>
> >> 'and ONLY Pentiums'
> >
> > It also says "should be included for any non-Pentium CPU that defines it"
> > but how do you know if the C# defines this? Is this another item that
> > would show up in the "Features" list? I looked at a PI-133 I have and
> > don't see it :
>
> It means that if you have 'cpu I586_CPU' in your kernel config and
> you aren't running a Pentium, then you can define this option.  However,
> it won't really buy you much since we only install the F00F hack once
> during bootup and all adding this option will do is just not check for
> it at bootup.

Cool, thanks for the clarification. I do not have I586_CPU defined in my
kernel conf (only I686_CPU) and had no problems building the kernel. Under
Linux you have to define -march=i586 to build the kernel properly for the
C3. I bring this up because you can specify CPUTYPE in the
/etc/defaults/make.conf. According to the notes in that file, the
following cpus can be defined :

#   Intel x86 architecture:
#       (AMD CPUs)      k7 k6-2 k6 k5
#       (Intel CPUs)    p4 p3 p2 i686 i586/mmx i586 i486 i386
#   Alpha/AXP architecture: ev6 pca56 ev56 ev5 ev45 ev4

Does CPUTYPE affect the -march flag? I ask because gnu apparently supports
the VIA C3 using -march=c3. As a test I set CPUTYPE=c3 and had no problems
building the kernel nor do I see any evidence that CPUTYPE was pulled from
make.conf during the kernel build. Is CPUTYPE only used for application
optimizations? I suspect that adding CPUTYPE=c3 was a misguided assumption
based on the gnu docs (see below).

http://gcc.gnu.org/onlinedocs/gcc-3.3.1/gcc/i386-and-x86-64-Options.html#i386%20and%20x86-64%20Options

Chris