[PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI Memory

[Linus Torvalds]

On Fri, Mar 2, 2018 at 8:57 AM, Linus Torvalds
<torvalds at linux-foundation.org> wrote:
>
> 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.

Side note: historically the two may have been almost the same, since
the CPU only had one single unified bus for "memory" (whether that was
memory-mapped PCI or actual RAM). The steering was external.

But even back then you had extended bits to specify things like how
the 640k-1M region got remapped - which could depend on not just the
address, but on whether you read or wrote to it.  The "lost" 384kB of
RAM could either be remapped at a different address, or could be used
for shadowing the (slow) ROM contents, or whatever.

              Linus