linux-kernel - Re: [PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI Memory

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Date:   Fri, 2 Mar 2018 09:34:42 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Kani, Toshi" <toshi.kani@....com>
Cc:     "benh@...nel.crashing.org" <benh@...nel.crashing.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "hch@....de" <hch@....de>, "axboe@...nel.dk" <axboe@...nel.dk>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "jglisse@...hat.com" <jglisse@...hat.com>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "maxg@...lanox.com" <maxg@...lanox.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "keith.busch@...el.com" <keith.busch@...el.com>,
        "oliveroh@....ibm.com" <oliveroh@....ibm.com>,
        "jgg@...pe.ca" <jgg@...pe.ca>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>
Subject: Re: [PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI Memory

On Fri, Mar 2, 2018 at 8:57 AM, Linus Torvalds
<torvalds@...ux-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