linux-kernel - Re: linux-next: manual merge of the xen-tip tree with the tip tree

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Date:	Wed, 12 Aug 2015 19:21:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:	Stephen Rothwell <sfr@...b.auug.org.au>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Xen Devel <Xen-devel@...ts.xensource.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	linux-next@...r.kernel.org, linux-kernel@...r.kernel.org,
	Andy Lutomirski <luto@...nel.org>,
	David Vrabel <david.vrabel@...rix.com>
Subject: Re: linux-next: manual merge of the xen-tip tree with the tip tree

On Wed, Aug 12, 2015 at 09:27:38AM -0400, Boris Ostrovsky wrote:

> Incidentally, 11276d53 ("locking/static_keys: Add a new static_key
> interface") breaks old-ish compilers (gcc version 4.4.4 20100503 (Red Hat
> 4.4.4-2) (GCC)):
> 
> 
> 
>   CC      arch/x86/kernel/nmi.o
> In file included from
> /home/build/linux-boris/include/linux/jump_label.h:109,
>                  from
> /home/build/linux-boris/arch/x86/include/asm/spinlock.h:5,
>                  from /home/build/linux-boris/include/linux/spinlock.h:88,
>                  from /home/build/linux-boris/arch/x86/kernel/nmi.c:14:
> /home/build/linux-boris/arch/x86/include/asm/jump_label.h: In function
> ‘nmi_handle’:
> /home/build/linux-boris/arch/x86/include/asm/jump_label.h:21: warning: asm
> operand 0 probably doesn’t match constraints
> /home/build/linux-boris/arch/x86/include/asm/jump_label.h:21: error:
> impossible constraint in ‘asm’
> make[3]: *** [arch/x86/kernel/nmi.o] Error 1
> make[2]: *** [arch/x86/kernel] Error 2
> make[1]: *** [arch/x86] Error 2

Ugh bugger.

I bet its that: &((char *)key)[branch] business, an earlier variant
thereof tripped up more recent GCCs too.

So its an __always_inline function, and both argument are always compile
time constants, @key is the address of an object in static storage (a
global) and @branch is a simple 0/1 at the call site.

Now we wish to compute (unsigned long)key + branch at compile/link time
to feed to the assembler as an immediate, which should be possible,
given its all 'constants'.

It just appears GCC is having a hard time with this.

Let me see if I have a sufficiently old GCC around to play with.
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