Re: [Xen-devel] [PATCH 2 of 6] x86: don't use .subsection to out-of-line failure path in spinlock.h

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.04.12 at 16:05, Tim Deegan <tim@xxxxxxx> wrote:
> At 14:28 +0100 on 05 Apr (1333636106), Jan Beulich wrote:
>> >>> On 05.04.12 at 14:07, Tim Deegan <tim@xxxxxxx> wrote:
>> > # HG changeset patch
>> > # User Tim Deegan <tim@xxxxxxx>
>> > # Date 1333626954 -3600
>> > # Node ID 0ecf439475e12f185553f42f56f099be5f328cce
>> > # Parent  8518fb0c8c996dca67efd39d31962a6d3502c2ed
>> > x86: don't use .subsection to out-of-line failure path in spinlock.h
>> > 
>> > LLVM's assembler doesn't support the .subsection directive.  Instead,
>> > leave the failure path inline with an unconditional jump past it in
>> > the success path (so the conditional jump is still forwards).
>> 
>> Please don't - the failure path should really be out-of-line here; that's
>> why the .subsection directive got added in the first place. And there
>> are more uses of .subsection elsewhere, which I hope you're not
>> intending to all undo just because of a shortcoming of a non-standard
>> assembler. 
> 
> The other uses are all in .S files, so don't go through clang's
> assembler.

Oh, so the compiler looks at the assembly itself? Or uses other than
gas for assembling (yet gas gets used for dealing with assembly files)?

>> Abstracting this in some way might be doable, e.g.
>> moving the code into .text.1 or some such when non-gas is used.
> 
> OK.  How about something like this?

This looks reasonable - if you package it so that with gcc it will be
.subsection and only with clang it would become .pushsection/
.popsection. (Also, to be generic, you may need to specify the
attributes of the .fixup section.)

Jan

> --- a/xen/include/asm-x86/spinlock.h  Thu Apr 05 14:51:31 2012 +0100
> +++ b/xen/include/asm-x86/spinlock.h  Thu Apr 05 15:03:31 2012 +0100
> @@ -45,11 +45,11 @@ static always_inline int _raw_read_trylo
>      asm volatile (
>          "    lock; decl %0         \n"
>          "    jns 2f                \n"
> -        "1:  .subsection 1         \n"
> +        "1:  .pushsection .fixup   \n"
>          "2:  lock; incl %0         \n"
>          "    decl %1               \n"
>          "    jmp 1b                \n"
> -        "    .subsection 0         \n"
> +        "    .popsection           \n"
>          : "=m" (rw->lock), "=r" (acquired) : "1" (1) : "memory" );
>  
>      return acquired;




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