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Subject: Re: [PATCH 2/2] x86, Makefile: Add new generic x86-64 settings
 v2/v3/v4
To:     Borislav Petkov <bp@alien8.de>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        "clang-built-linux@googlegroups.com" 
        <clang-built-linux@googlegroups.com>,
        "graysky@archlinux.us" <graysky@archlinux.us>,
        "masahiroy@kernel.org" <masahiroy@kernel.org>,
        Christoph Hellwig <hch@infradead.org>
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From:   Tor Vic <torvic9@mailbox.org>
Message-ID: <25f1e81a-84f7-e63c-2989-cd4fddd695e7@mailbox.org>
Date:   Wed, 18 Aug 2021 18:28:47 +0000
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On 18.08.21 17:59, Borislav Petkov wrote:
> On Wed, Aug 18, 2021 at 05:08:40PM +0200, torvic9@mailbox.org 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^

That's something I'd like to help with, hence my (not very good)
submissions.
You're one of the kernel colonels, so I'm happy to get some *useful*
feedback which enables noobs like me to get a better understanding of
the kernel stuff.
Also, credits to graysky because this is actually based on his work.

> 
> Thx.
>