linux-kernel - Re: [PATCH 01/22] ARM: add mechanism for late code patching

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Message-ID: <20120806111224.GA18957@n2100.arm.linux.org.uk>
Date:	Mon, 6 Aug 2012 12:12:24 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Cyril Chemparathy <cyril@...com>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	arnd@...db.de, catalin.marinas@....com, nico@...aro.org,
	will.deacon@....com
Subject: Re: [PATCH 01/22] ARM: add mechanism for late code patching

On Tue, Jul 31, 2012 at 07:04:37PM -0400, Cyril Chemparathy wrote:
> +static void __init init_patch_kernel(void)
> +{
> +	const void *start = &__patch_table_begin;
> +	const void *end   = &__patch_table_end;
> +
> +	BUG_ON(patch_kernel(start, end - start));
> +	flush_icache_range(init_mm.start_code, init_mm.end_code);

Err.  You are asking the kernel to flush every single cache line
manually throughout the kernel code.  That's a flush every 32-bytes
over maybe a few megabytes of address space.

This is one of the reasons we do the patching in assembly code before
the caches are enabled - so we don't have to worry about the interaction
with the CPU caches, which for this kind of application would be very
expensive.
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