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

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Message-ID: <501FC8CA.20303@ti.com>
Date:	Mon, 6 Aug 2012 09:38:18 -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 9:26 AM, Russell King - ARM Linux wrote:
> On Mon, Aug 06, 2012 at 09:19:10AM -0400, Cyril Chemparathy wrote:
>> With a flush_cache_all(), we could avoid having to operate a cacheline
>> at a time, but that clobbers way more than necessary.
>
> You can't do that, because flush_cache_all() on some CPUs requires the
> proper MMU mappings to be in place, and you can't get those mappings
> in place because you don't have the V:P offsets fixed up in the kernel.
> Welcome to the chicken and egg problem.
>
>> 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.
>
> There we disagree on a few orders of magnitude.  There are relatively
> few places that need updating.  According to the kernel I have here:
>
>     text    data     bss     dec     hex filename
> 7644346  454320  212984 8311650  7ed362 vmlinux
>
> Idx Name          Size      VMA       LMA       File off  Algn
>    1 .text         004cd170  c00081c0  c00081c0  000081c0  2**5
>   16 .init.pv_table 00000300  c0753a24  c0753a24  00753a24  2**0
>
> That's about 7MB of text, and only 192 points in that code which need
> patching.  Even if we did this with caches on, that's still 192 places,
> and only 192 places we'd need to flush a cache line.
>
> Alternatively, with your approach and 7MB of text, you need to flush
> 238885 cache lines to cover the entire kernel.
>
> It would be far _cheaper_ with your approach to flush the individual
> cache lines as you go.
>

Agreed.  Thanks.

-- 
Thanks
- Cyril
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