linux-kernel - Re: [PATCH v3 01/11] PCI/P2PDMA: Support peer-to-peer memory

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Date:   Wed, 14 Mar 2018 10:23:27 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     David Laight <David.Laight@...LAB.COM>,
        Sinan Kaya <okaya@...eaurora.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>
Cc:     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>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH v3 01/11] PCI/P2PDMA: Support peer-to-peer memory



On 14/03/18 06:16 AM, David Laight wrote:
> That surprises me (unless I missed something last time I read the spec).
> While P2P writes are relatively easy to handle, reads and any other TLP that
> require acks are a completely different proposition.
> There are no additional fields that can be set in the read TLP and will be
> reflected back in the ack(s) than can be used to route the acks back to the
> correct initiator.
>
> I'm pretty sure that to support P2P reads a switch would have to save
> the received read TLP and (possibly later on) issue read TLP of its own
> for the required data.
> I'm not even sure it is easy to interleave the P2P reads with those
> coming from the root.
> That requires a potentially infinite queue of pending requests.

This is wrong. A completion is a TLP just like any other and makes use
of the Destination ID field in the header to route it back to the
original requester.

> Some x86 root ports support P2P writes (maybe with a bios option).
> It would be a shame not to be able to do P2P writes on such systems
> even though P2P reads won't work.

Yes, and this has been discussed many times. It won't be changing in the
near term.

Logan