linux-kernel - Re: [RFC] gcc feature request: Moving blocks into sections

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Date:	Mon, 5 Aug 2013 18:08:37 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>, gcc <gcc@....gnu.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Daney <ddaney.cavm@...il.com>,
	Behan Webster <behanw@...verseincode.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [RFC] gcc feature request: Moving blocks into sections

* Steven Rostedt (rostedt@...dmis.org) wrote:
> On Mon, 2013-08-05 at 17:28 -0400, Mathieu Desnoyers wrote:
> 
[...]
> > My though is that the code above does not cover all jump encodings that
> > can be generated by past, current and future x86 assemblers.
> > 
> > Another way around this issue might be to keep the instruction size
> > within a non-allocated section:
> > 
> > static __always_inline bool arch_static_branch(struct static_key *key)
> > {
> >         asm goto("1:"
> >                 "jmp %l[l_yes]\n\t"
> >                 "2:"
> > 
> >                 ".pushsection __jump_table,  \"aw\" \n\t"
> >                 _ASM_ALIGN "\n\t"
> >                 _ASM_PTR "1b, %l[l_yes], %c0 \n\t"
> >                 ".popsection \n\t"
> > 
> >                 ".pushsection __jump_table_ilen \n\t"
> >                 _ASM_PTR "1b \n\t"      /* Address of the jmp */
> >                 ".byte 2b - 1b \n\t"    /* Size of the jmp instruction */
> >                 ".popsection \n\t"
> > 
> >                 : :  "i" (key) : : l_yes);
> >         return false;
> > l_yes:
> >         return true;
> > }
> > 
> > And use (2b - 1b) to know what size of no-op should be used rather than
> > to rely on instruction decoding.
> > 
> > Thoughts ?
> > 
> 
> Then we need to add yet another table of information to the kernel that
> needs to hang around. This goes with another kernel-discuss request
> talking about kernel data bloat.

Perhaps this section could be simply removed by the post-link stage ?

Thanks,

Mathieu

> 
> -- Steve
> 
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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