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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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