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Re: [Xen-devel] [PATCH v2 02/12] x86/alternatives: allow using assembler macros in favor of C ones


  • To: Jan Beulich <JBeulich@xxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Wed, 29 Aug 2018 15:52:42 +0100
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  • Delivery-date: Wed, 29 Aug 2018 14:53:12 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 29/08/18 15:00, Jan Beulich wrote:
> As was validly pointed out as motivation for similar Linux side changes
> (https://lkml.org/lkml/2018/6/22/677), using long sequences of
> directives and auxiliary instructions, like is commonly the case when
> setting up an alternative patch site, gcc can be mislead into believing
> an asm() to be more heavy weight than it really is. By presenting it
> with an assembler macro invocation instead, this can be avoided.
>
> Initially I wanted to outright change the C macros ALTERNATIVE() and
> ALTERNATIVE_2() to invoke the respective assembler ones, but doing so
> would require quite a bit of cleanup of some use sites, because of the
> exra necessary quoting combined with the need that each assembler macro
> argument must consist of just a single string literal. We can consider
> working towards that subsequently.
>
> For now, set the stage of using the assembler macros here by providing a
> new generated header, being the slightly massaged pre-processor output
> of (for now just) alternative-asm.h. The massaging is primarily to be
> able to properly track the build dependency: For this, we need the C
> compiler to see the inclusion, which means we shouldn't directly use an
> asm(". include ...") directive.
>
> The dependency added to asm-offsets.s is not a true one; it's just the
> easiest approach I could think of to make sure the new header gets
> generated early on, without having to fiddle with xen/Makefile (and
> introducing some x86-specific construct there).
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

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