On Fri, Mar 2, 2018 at 8:22 AM, Kani, Toshi <toshi.k...@hpe.com> wrote: > > FWIW, this thing is called MTRRs on x86, which are initialized by BIOS.
No. Or rather, that's simply just another (small) part of it all - and an architected and documented one at that. Like the page table caching entries, the memory type range registers are really just "secondary information". They don't actually select between PCIe and RAM, they just affect the behavior on top of that. The really nitty-gritty stuff is not architected, and generally not documented outside (possibly) the BIOS writer's guide that is not made public. Those magical registers contain details like how the DRAM is interleaved (if it is), what the timings are, where which memory controller handles which memory range, and what are goes to PCIe etc. Basically all the actual *steering* information is very much hidden away from the kernel (and often from the BIOS too). The parts we see at a higher level are just tuning and tweaks. Note: the details differ _enormously_ between different chips. The setup can be very different, with things like Knights Landing having the external cache that can also act as local memory that isn't a cache but maps at a different physical address instead etc. That's the kind of steering I'm talking about - at a low level how physical addresses get mapped to different cache partitions, memory controllers, or to the IO system etc. Linus