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

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Date:	Mon, 6 Aug 2012 09:19:10 -0400
From:	Cyril Chemparathy <cyril@...com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
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 8/6/2012 7:12 AM, Russell King - ARM Linux wrote:
> 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.
>

With a flush_cache_all(), we could avoid having to operate a cacheline 
at a time, but that clobbers way more than necessary.

Maybe the better answer is to flush only the patched cachelines.

> 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.
>

Sure, flushing caches is expensive.  But then, so is running the 
patching code with caches disabled.  I guess memory access latencies 
drive the performance trade off here.

-- 
Thanks
- Cyril
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