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SubjectRe: [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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