linux-kernel - Re: [PATCH 2/4] x86/xor: improve XMM register spill/fill

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Date:	Mon, 10 Sep 2012 15:46:36 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	<mingo@...e.hu>, <tglx@...utronix.de>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] x86/xor: improve XMM register spill/fill

>>> On 10.09.12 at 16:05, "H. Peter Anvin" <hpa@...or.com> wrote:
> On 09/10/2012 05:38 AM, Jan Beulich wrote:
>> +/*
>> + * By forcing the alignment beyond the default of 16 bytes, we make the
>> + * compiler guarantee the alignment. Passing -mincoming-stack-boundary=3
>> + * (which would have been the better global alternative, as the kernel
>> + * never guarantees better stack alignment) isn't permitted on x86-64.
>> + */
> 
> The very latest gcc should handle it, and in fact we compile with 
> -mstack-alignment=3 if gcc accepts it (if it is not yet upstream it will 
> be soon.)  This affects the validity of this patch.

The comment would be stale with that, but the code should still
be fine - it would merely over-align the stack in that case (to
32 bytes when 16 would suffice). Or did you spot something else
that I'm missing?

Also, I can't spot any use of -mstack-alignment= in today's tip's
arch/x86/Makefile* - where's that hidden?

Jan

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