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

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
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