Re: [PATCH 2/2] x86, Makefile: Add new generic x86-64 settings v2/v3/v4

From: Borislav Petkov
Date: Wed Aug 18 2021 - 13:58:33 EST

On Wed, Aug 18, 2021 at 05:08:40PM +0200, torvic9@xxxxxxxxxxx wrote:
> I ran some quick checks and benchmarks, and your doubts seem to be
> justified. A 5.14-rc6 kernel compiled with the default 'generic' and
> one built with 'x86-64-v3' even have the exact same uncompressed file
> size. Benchmarks were inconclusive as well.

Lemme preface this with a IMHO:

Yeah, those -march machine-specific compiler switches don't really
show any perf improvements for kernels because, well, if you look at
the instruction stream a kernel executes, there's not really a whole
lot left to optimize after -O2.

Also, the percentage of time a machine spends in the kernel should be a
lot smaller (orders of magnitude) than in userspace - the operative word
being *should* here :-) - so there really isn't anything to optimize.

Not to say that there aren't a gazillion other places in the kernel that
could use more eyes and testing. ^Hint hint^