Lazy D-cache flush (non aliasing VIPT)

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本文探讨了Linux内核中的flush_dcache_page()函数如何确保内核与用户空间的一致性视图。该函数主要在内核写入可能映射到用户空间的页面时调用，以清除内核映射；或在读取具有用户空间映射的页面前调用，以保持内核和用户空间映射的一致性。对于非别名VIPT缓存，通常不需要刷新缓存，除非涉及到代码页的映射。

Linux Kernel / Linux Kernel - Vineet Gupta [synopsys.com] - 7 May 2013 08:38 EDT

flush_dcache_page( ) is MM hook to ensure that a page has consistent
views between kernel and userspace. Thus it is called when

- kernel writes to a page which at some later point could get mapped to userspace (so kernel mapping needs to be flushed-n-inv)
- kernel is about to read from a page with possible userspace mappings (so userspace mappings needs to be made coherent with kernel ones)

However for Non aliasing VIPT dcache, any userspace mapping will always be congruent to kernel mapping. Thus d-cache need need not be flushed at all (or delayed indefinitely).

The only reason it does need to be flushed is when mapping code pages. Since icache doesn't snoop dcache, those dirty dcache lines need to be written back to memory and icache line invalidated so that icache lines fetch will get the right data.

Decent gains on LMBench fork/exec/sh and File I/O micro-benchmarks.