linux-kernel - Re: [PATCH v0 1/5] x86_64: march=native support

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Message-ID: <CACVxJT8xcR8Apg3sOKEaa92OMdTrphzx_tPTZAnq8ptqgOjLnw@mail.gmail.com>
Date:   Fri, 8 Dec 2017 11:57:20 +0200
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
        mingo@...hat.com
Subject: Re: [PATCH v0 1/5] x86_64: march=native support

On 12/8/17, H. Peter Anvin <hpa@...or.com> wrote:
> One more thing: you HAVE to make
> arch/x86/include/asm/required-features.h aware of any features that the
> kernel unconditionally depend on.

Yes, this is foolprof part I have to think through.

> Again, using the gcc cpp macros that reflect what bits gcc itself
> broadcast.  However, this is perhaps where CONFIG flags become
> important, since required-features.h has to be able to be compiled in
> the bootcode environment, which is different from the normal kernel
> compiler environment.
>
> We could, however, automagically generate a reflection of these as a
> header file:
>
> 	echo '#ifndef __LINUX_CC_DEFINES__'
> 	echo '#define __LINUX_CC_DEFINES__'
> 	$(CC) $(c_flags) -x c -E -Wp,-dM /dev/null | sort | \
> 	sed -nr -e 's/^#define __([^[:space]]+)__$/#define __KERNEL_CC_\1/p'
> 	echo '#endif

A lot of then aren't interesting and duplicate each other.
Another thing: clang. It detects machine I'm typing this as __corei7__
while gcc does it as __core_avx2__.