linux-kernel - Re: [RFC PATCH 0/6] jump label v3

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Date:	Wed, 18 Nov 2009 15:07:58 -0800 (PST)
From:	Roland McGrath <roland@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Jason Baron <jbaron@...hat.com>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, mathieu.desnoyers@...ymtl.ca, tglx@...utronix.de,
	rostedt@...dmis.org, andi@...stfloor.org, rth@...hat.com,
	mhiramat@...hat.com
Subject: Re: [RFC PATCH 0/6] jump label v3

> 67 66 8D 74 00 (lea si,[si+0]) should work as a 32-bit atomic NOP.  It's
> not necessarily the fastest, though (I have no data on that.)
> Similarly, 66 66 66 66 90 should also work.

We should get all the knowledge like that stored in places like the
Kconfig.cpu comments near X86_P6_NOP and/or asm/nops.h macros and comments.

Let's have an ASM_ATOMIC_NOP5 macro in asm/nops.h?  I've lost track of the
variants, and I'll leave that to you all who are close to the chip people.

I can't tell if it's the case that there will be kernel configurations
where there is one known-safe choice but a different choice that's optimal
if CPU model checks pass.  If so, we could compile in the safe choice and
then mass-change to the optimal choice at boot time.  (i.e. like the
alternatives support, but we don't really need to use that too since we
already have another dedicated table of all the PC addresses to touch.)

Thanks,
Roland
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