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

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Date:   Thu, 1 Mar 2018 13:53:15 -0700
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Benjamin Herrenschmidt <benh@....ibm.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pci@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-rdma <linux-rdma@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        linux-block@...r.kernel.org, Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Oliver OHalloran <oliveroh@....ibm.com>
Subject: Re: [PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI Memory

On Fri, Mar 02, 2018 at 07:40:15AM +1100, Benjamin Herrenschmidt wrote:
> Also we need to be able to hard block MEMREMAP_WB mappings of non-RAM
> on ppc64 (maybe via an arch hook as it might depend on the processor
> family). Server powerpc cannot do cachable accesses on IO memory
> (unless it's special OpenCAPI or nVlink, but not on PCIe).

I think you are right on this - even on x86 we must not create
cachable mappings of PCI BARs - there is no way that works the way
anyone would expect.

I think this series doesn't have a problem here only because it never
touches the BAR pages with the CPU.

BAR memory should be mapped into the CPU as WC at best on all arches..

Jason