linux-kernel - Re: [patch 5/9] Conditional Calls

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Date:	31 May 2007 15:54:32 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 5/9] Conditional Calls - i386 Optimization

Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> writes:

> +#define cond_call_optimized(flags, name, func) \
> +	({ \
> +		static const char __cstrtab_name_##name[] \
> +		__attribute__((section("__cond_call_strings"))) = #name; \
> +		char condition; \
> +		asm (	".section __cond_call, \"a\", @progbits;\n\t" \
> +					".long %1, 0f, %2;\n\t" \
> +					".previous;\n\t" \
> +					".align 2\n\t" \
> +					"0:\n\t" \
> +					"movb %3,%0;\n\t" \
> +				: "=r" (condition) \
> +				: "m" (*__cstrtab_name_##name), \
> +				  "m" (*(char*)flags), \
> +				  "i" ((flags) & CF_STATIC_ENABLE)); \

Remind me what we need the flags again for? 

I would prefer to just eliminate them and always require arming.


> +		(likely(!condition)) ? \
> +			(__typeof__(func))0 : \
> +			(func); \
> +	})
> +
> +/* cond_call macro selecting the generic or optimized version of cond_call,
> + * depending on the flags specified. */
> +#define _cond_call(flags, name, func) \
> +({ \
> +	(((flags) & CF_LOCKDEP) && ((flags) & CF_OPTIMIZED)) ? \

Similar here? unoptimized condcalls don't make much sense.
I also don't understand what the LOCKDEP flag is good for.

> Index: linux-2.6-lttng/arch/i386/kernel/condcall.c
> ===================================================================
> --- /dev/null	1970-01-01 00:00:00.000000000 +0000
> +++ linux-2.6-lttng/arch/i386/kernel/condcall.c	2007-05-17 01:52:38.000000000 -0400
> @@ -0,0 +1,140 @@
> +/* condcall.c
> + *
> + * - Erratum 49 fix for Intel PIII.
> + * - Still present on newer processors : Intel Core 2 Duo Processor for Intel
> + *   Centrino Duo Processor Technology Specification Update, AH33.
> + *   Unsynchronized Cross-Modifying Code Operations Can Cause Unexpected
> + *   Instruction Execution Results.

Can you please define a generic utility function to do self modifying
code? There are other places that do it too.

> +static DEFINE_MUTEX(cond_call_mutex);

All locks need a comment describing what they protect and the hierarchy if 
any.

-Andi
-
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