mm/highmem: make kmap cache coloring aware
User-visible effect: Architectures that choose this method of maintaining cache coherency (MIPS and xtensa currently) are able to use high memory on cores with aliasing data cache. Without this fix such architectures can not use high memory (in case of xtensa it means that at most 128 MBytes of physical memory is available). The problem: VIPT cache with way size larger than MMU page size may suffer from aliasing problem: a single physical address accessed via different virtual addresses may end up in multiple locations in the cache. Virtual mappings of a physical address that always get cached in different cache locations are said to have different colors. L1 caching hardware usually doesn't handle this situation leaving it up to software. Software must avoid this situation as it leads to data corruption. What can be done: One way to handle this is to flush and invalidate data cache every time page mapping changes color. The other way is to always map ph...
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