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

[Andy Lutomirski]

On Nov 7, 2015 3:21 AM, "Thomas Gleixner" <tglx@xxxxxxxxxxxxx> wrote:
>
> On Fri, 6 Nov 2015, Andy Lutomirski wrote:
>
> > Unfortunately, these only work if HAVE_JUMP_LABEL.  In principle, we
> > could do some serious surgery on the core jump label infrastructure
> > to keep the patch infrastructure available on x86 on all builds, but
> > that's probably not worth it.
>
> No, but we can be smarter about it. There is nothing which that asm
> entry code needs from linux/jump_label.h and asm/jump_label.h execpt
> STATIC_KEY_INIT_NOP. Something like the patch below should do the
> trick.

I like some bits of this.  See below.

>
> Thanks,
>
>         tglx
>
> Index: tip/arch/x86/entry/calling.h
> ===================================================================
> --- tip.orig/arch/x86/entry/calling.h
> +++ tip/arch/x86/entry/calling.h
> @@ -232,3 +232,22 @@ For 32-bit we have the following convent
>
>  #endif /* CONFIG_X86_64 */
>
> +/* 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.)

> +       1:
> +       jmp.d32         2f

Old binutils will choke on this.  But maybe no one has jump labels and
old binutils.

> +       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.

> +#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.

--Andy
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