Ralf Baechle (ralf@xxxxxxxxxxxxxx) writes: > > A truly safe and general I-cache flush routine should itself run > > uncached... It depends what you mean by general, and uncached is not the only option. The spec says: "The operation of the instruction is UNPREDICTABLE if the cache line that contains the CACHE instruction is the target of an invalidate..." If you use hit-type cache operations in a kernel routine, then you're safe. I can't envisage any circumstance in which Linux would try to invalidate kernel mainline code locations from the I-cache (well, you might be doing something fabulous with debugging the kernel, but that's not normal and you'd hardly expect to be able to support such an activity with standard cache management calls). So this problem can only arise on index-type I-cache invalidation. I claim that a running kernel on a MIPS CPU should only use index-type invalidation when it is necessary to invalidate the entire I-cache. (If you use index-type operations for a range which doesn't resolve to "the whole cache" then that should be fixed). That implies that a MIPS32-paranoid "invalidate-whole-I-cache" routine should: 1. Identify which indexes might alias to cache lines containing the routines's own 'cache invalidate' instruction(s), and thus hit the problem. There won't be that many of them. 2. Arrange to skip those indexes when zapping the cache, then do something weird to invalidate that handful of lines. You could do that by running uncached, but you could also do it just by using some auxiliary routine which is known to be more than a cache line but much less than a whole I-cache span distant, so can't possibly alias to the same thing... This is fiddly, but not terribly difficult and should have a negligible performance impact. Does that make sense? Am I now, having named the solution, responsible for figuring out a patch (yeuch, I never wanted to be a kernel programmer again...). -- Dominic Sweetman MIPS Technologies
