linux-kernel - Re: [RFC 1/8] x86: objtool: use asm macro for better compiler decisions

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Date:   Tue, 15 May 2018 14:55:41 -0700
From:   Josh Triplett <josh@...htriplett.org>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     linux-kernel@...r.kernel.org, Christopher Li <sparse@...isli.org>,
        linux-sparse@...r.kernel.org
Subject: Re: [RFC 1/8] x86: objtool: use asm macro for better compiler
 decisions

On Tue, May 15, 2018 at 02:53:52PM -0700, Nadav Amit wrote:
> Josh Triplett <josh@...htriplett.org> wrote:
> 
> > On Tue, May 15, 2018 at 07:11:08AM -0700, Nadav Amit wrote:
> >> GCC considers the number of statements in inlined assembly blocks,
> >> according to new-lines and semicolons, as an indication to the cost of
> >> the block in time and space. This data is distorted by the kernel code,
> >> which puts information in alternative sections. As a result, the
> >> compiler may perform incorrect inlining and branch optimizations.
> >> 
> >> In the case of objtool, this distortion is extreme, since anyhow the
> >> annotations of objtool are discarded during linkage.
> >> 
> >> The solution is to set an assembly macro and call it from the inlinedv
> >> assembly block. As a result GCC considers the inline assembly block as
> >> a single instruction.
> >> 
> >> This patch slightly increases the kernel size.
> >> 
> >>   text    data     bss     dec     hex filename
> >> 18126699 10066728 2936832 31130259 1db0293 ./vmlinux before
> >> 18126824 10067268 2936832 31130924 1db052c ./vmlinux after (+665)
> > 
> > With what kernel config? What's the text size change for an allnoconfig?
> 
> I use my custom config and to include the drivers in the image. Other
> configs were not too appropriate: defconfig does not enable paravirt and
> allyesconfig is too heavy.
> 
> There is no effect with allnoconfig on size:
> 
>    text	   data	    bss	    dec	    hex	filename
> 1018441	 230148	1215604	2464193	 2599c1	./vmlinux
> 1018441	 230148	1215604	2464193	 2599c1	./vmlinux
> 
> I think it is expected since CONFIG_STACK_VALIDATION is off. No?

Thanks for checking; fine by me then. :)