linux-kernel - Re: [PATCH 3/4] x86/asm: Add asm macros for static keys/jump labels

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Date:	Sat, 7 Nov 2015 17:58:14 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andy Lutomirski <luto@...capital.net>
cc:	Borislav Petkov <bp@...en8.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Brian Gerst <brgerst@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 3/4] x86/asm: Add asm macros for static keys/jump
 labels

On Sat, 7 Nov 2015, Andy Lutomirski wrote:
> On Nov 7, 2015 3:21 AM, "Thomas Gleixner" <tglx@...utronix.de> wrote:
> > +/* ASM jump label support */
> > +#if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
> > +#include <asm/jump_label.h>
> > +
> > +       .macro STATIC_CALL_IF_ENABLED fun, key
> 
> I think we still need the "def" parameter.  But maybe the "static
> call" is a good idea.  (By giving 'def' a default value or omitting it
> entirely, it's too easy not to think about it, and that's a huge
> historical source of breakage and it's a good part of the reason for
> the new improved C API.  At least these days I *think* we'll diagnose
> the problem on bootup instead of waiting for random breakage later.)

Ok. That's easy to add :)
 
> > +       1:
> > +       jmp.d32         2f
> 
> Old binutils will choke on this.  But maybe no one has jump labels and
> old binutils.

Good question.
 
> > +       call            fun
> > +       .pushsection __jump_table, "aw"
> > +       _ASM_ALIGN
> > +       _ASM_PTR        1b, 2f, \key
> > +       .popsection
> > +       2:
> > +       .endm
> > +#else
> > +       .macro STATIC_CALL_IF_ENABLED fun, key
> > +       call            fun
> > +       .endm
> 
> This could result in errors down the road, since this macro is really
> "static call if enabled or if the kernel is built without jump
> labels".  enter_from_user_mode is special because it's already a no-op
> if context tracking is off, but I'm not sure that this oddity should
> leak into more generic code.

Hmm. Maybe we need a better name for this macro.
 
> > +#endif
> > Index: tip/arch/x86/entry/entry_64.S
> > ===================================================================
> > --- tip.orig/arch/x86/entry/entry_64.S
> > +++ tip/arch/x86/entry/entry_64.S
> > @@ -510,7 +510,7 @@ END(irq_entries_start)
> >          */
> >         SWAPGS
> >  #ifdef CONFIG_CONTEXT_TRACKING
> > -       call enter_from_user_mode
> > +       STATIC_CALL_IF_ENABLED enter_from_user_mode, context_tracking_enabled
> >  #endif
> 
> This doesn't get rid of the ifdeffery, which I thought was a nice
> feature of my patch.

Right. I was just too lazy to add another macro :)

My main point was to avoid the extra stuff for !HAVE_JUMP_LABEL and
hide this in a header file.

Thanks,

	tglx


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