linux-kernel - Re: gas 2.16 and assembly macros -- entry

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Date:	Fri, 01 Oct 2010 08:21:58 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jan Beulich <JBeulich@...ell.com>
CC:	Ingo Molnar <mingo@...e.hu>, heukelum@...tmail.fm,
	tglx@...utronix.de, akpm@...ux-foundation.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: gas 2.16 and assembly macros -- entry_64.S build failure

On 10/01/2010 01:27 AM, Jan Beulich wrote:
>>>> On 01.10.10 at 02:26, "H. Peter Anvin" <hpa@...or.com> wrote:
>> ... but that doesn't work with the macros like movq_cfi.  On those, we
> 
> Is that only because of the register names used as operands to
> movq_cfi etc not having the % specified right away? I don't think
> that is really needed, i.e. the % could go there rather than being
> added in the macro body - the .cfi_* directives are perfectly happy
> with having the prefix there (and I don't know why it was coded
> this way in the first place, as this made it less similar to the plain
> movq while I thought the goal was to keep the differences to a
> minimum).
> 

No, and in fact the problem that spurred this discussion was in the use
of immediates, not registers:

	pushq_cfi $(USER_DS)

Obviously we can't add the % register prefix, since pushq can take
either a register or an immediate (or, for that matter, a memory operand).

>> could argue that at least people won't put $ on them, but cpp will still
>> split them apart with spaces; this apparently causes problems at least
>> as soon as there is an expression more complicated than addition
>> involved (apparently plus signs are okay, but minus signs aren't!)
>>
>> I'm completely lost about how to deal with this.   We can't simply
>> defang the macros -- at least not in a way that is likely to *stay*
>> working -- and dropping the macros is seriously going to impact the
>> debuggability of the kernel.  One way, of course, is to simply declare
>> binutils 2.16 and 2.15.9x (which is apparently included in
>> RHEL/CentOS 4) to be broken beyond repair unless distros backport a fix,
>> and in many ways I think that is the preferred option, but I don't know
>> if that makes sense to others...
> 
> The other alternative, albeit disliked by Ingo, continues to be to use
> __stringify() on all non-trivial operands, which then wouldn't require
> suppressing CONFIG_AS_CFI for pre-2.17 binutils.

You should be taken out and shot for even thinking that, never mind
putting it in writing...

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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