kernel cpu entries (was: Odd performance problems after upgrade from 4.11 to 6.0-Stable)

[Jonathan Noack]

Kevin Oberman wrote:
> Scott Long wrote:
>> Also, taking out CPU_I586 is usually a bad idea.  It offers no 
>> performance penalties (unlike CPU_I386 and maybe CPU_I486), but
>> enables things like optimized bcopy.
> 
> Ahh, This is the sort of thing I never realized. Is there anything in
> the handbook that covers this? I had always been under the impression
> that CPU_I686 enabled all things that the 686 was capable of. I will
> build a new kernel to add that back in.

 From tuning(7):
**************************************************
There are a number of *_CPU options that can be commented out.  If you 
only want the kernel to run on a Pentium class CPU, you can easily 
remove I486_CPU, but only remove I586_CPU if you are sure your CPU is 
being recognized as a Pentium II or better.  Some clones may be 
recognized as a Pentium or even a 486 and not be able to boot without 
those options.  If it works, great!  The operating system will be able 
to better use higher-end CPU features for MMU, task switching, timebase, 
and even device operations...
**************************************************

 From /sys/i386/conf/NOTES:
**************************************************
# You must specify at least one CPU (the one you intend to run on);
# deleting the specification for CPUs you don't need to use may make
# parts of the system run faster.
**************************************************

 From npx(4) (also see /sys/i386/i386/support.s):
**************************************************
The NPX registers are normally used to optimize copying and zeroing when 
all of the following conditions are satisfied:
1.   cpu I586_CPU is an option
...
Then copying and zeroing using the NPX registers is normally 30-100% faster.
**************************************************

All is rosy until you see that I586_CPU looks like a loss for blowfish 
(if you have an i686 CPU):
/sys/crypto/blowfish/arch/i386/bf_enc.S

As I use AES, I guess I586_CPU is a win for me.  Despite this, I think 
it makes the most sense for I686_CPU to enable the optimized bcopy if it 
really is a win for i686 CPUs.

-Jonathan