Kernel Optimizations for Processors

[John Baldwin]

On 29-Dec-2003 admin at forkthepenguin.com wrote:
> 
> 
> 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).

CPUTYPE does affect the -march flag, yes.  However, /usr/share/mk/bsd.cpu.mk
currently doesn't support the c3 CPU type for i386.  If you want to add support
for it and submit a patch in a PR, that would be great.

-- 

John Baldwin <jhb at FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve!"  -  http://www.FreeBSD.org/