Yes, but I want it to be guaranteed, not statistically. E.g. I want to run Quake1 right from the L3 cache. E.g. with filesystem I'm sure when I read or write

Yes, but I want it to be guaranteed, not statistically. E.g. I want to run Quake1 right from the L3 cache. E.g. with filesystem I'm sure when I read or write it, if I use regular IO operations, no mmap-ing (of course there's a lot of quirks with drive caches not respecting fsync, etc, but let's skip that for sake of this comment).