linux-kernel - Re: PROPOSAL: Extend inline asm syntax with size spec

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Message-ID: <alpine.LSU.2.20.1810101034020.16707@zhemvz.fhfr.qr>
Date:   Wed, 10 Oct 2018 10:35:54 +0200 (CEST)
From:   Richard Biener <rguenther@...e.de>
To:     Borislav Petkov <bp@...en8.de>
cc:     Segher Boessenkool <segher@...nel.crashing.org>,
        Ingo Molnar <mingo@...nel.org>, Michael Matz <matz@...e.de>,
        gcc@....gnu.org, Nadav Amit <namit@...are.com>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Sam Ravnborg <sam@...nborg.org>,
        Alok Kataria <akataria@...are.com>,
        Christopher Li <sparse@...isli.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H. Peter Anvin" <hpa@...or.com>, Jan Beulich <JBeulich@...e.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>,
        linux-sparse@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        virtualization@...ts.linux-foundation.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        linux-xtensa@...ux-xtensa.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: PROPOSAL: Extend inline asm syntax with size spec

On Wed, 10 Oct 2018, Borislav Petkov wrote:

> On Wed, Oct 10, 2018 at 03:03:25AM -0500, Segher Boessenkool wrote:
> > The code immediately after this makes it size 1, even for things like
> > asm(""), I suppose this works better for the inliner.  But that's a detail
> > (and it might change); the description says "consider this asm as minimum
> > length and cost for inlining decisions", which works for either 0 or 1.
> 
> Thanks for implementing this, much appreciated. If you need people to
> test stuff, lemme know.
> 
> > You can think of it as meaning "we want this asm inlined always", and then
> > whether that actually happens depends on if the function around it is
> > inlined or not.
> 
> My only concern is how we would catch the other extremity where the
> inline asm grows too big and we end up inlining it everywhere and thus
> getting fat. The 0day bot already builds tinyconfigs but we should be
> looking at vmlinux size growth too.

Well, it's like always-inline functions, you have to be careful
and _not_ put it everywhere...  It's also somewhat different to
always-inline functions as those lose their special-ness once
inlined (the inlined body is properly accounted for size).

Richard.