linux-kernel - Re: [PATCH v6 06/13] PCI/P2PDMA: Add P2P DMA driver writer's documentation

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Date:   Fri, 21 Sep 2018 11:41:26 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-rdma@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-block@...r.kernel.org,
        Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        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>,
        Christian König <christian.koenig@....com>,
        Jens Axboe <axboe@...nel.dk>, Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH v6 06/13] PCI/P2PDMA: Add P2P DMA driver writer's
 documentation

On Wed, Sep 12, 2018 at 06:11:49PM -0600, Logan Gunthorpe wrote:
> Add a restructured text file describing how to write drivers
> with support for P2P DMA transactions. The document describes
> how to use the APIs that were added in the previous few
> commits.
> 
> Also adds an index for the PCI documentation tree even though this
> is the only PCI document that has been converted to restructured text
> at this time.
> 
> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>

Acked-by: Bjorn Helgaas <bhelgaas@...gle.com>

> +With the client list in hand, the orchestrator may then call
> +:c:func:`pci_p2pmem_find()` to obtain a published P2P memory provider
> +that is supported (behind the same root port) as all the clients. If more
> +than one provider is supported, the one nearest to all the clients will
> +be chosen first. If there are more than one provider is an equal distance

s/If there are more/If more/

> +away, the one returned will be chosen at random. This function returns the PCI

s/the one returned will be chosen at random/one will be chosen
arbitrarily/ ?  (I doubt it's really random)

> +device to use for the provider with a reference taken and therefore
> +when it's no longer needed it should be returned with pci_dev_put().

> +Struct Page Caveats
> +-------------------
> +
> +Driver writers should be very careful about not passing these special
> +struct pages to code that isn't prepared for it. At this time, the kernel
> +interfaces do not have any checks for ensuring this. This obviously
> +precludes passing these pages to userspace.

Sounds like landmines here since the reader probably can't translate
"code that isn't prepared for it" into a list of interfaces that are
off-limits.  But that's a VM issue that is above my pay grade, so I'm
not suggesting any change; just pointing out something that makes me
wonder "hmmm..., how would I act on this?"

> +P2P memory is also technically IO memory but should never have any side
> +effects behind it. Thus, the order of loads and stores should not be important
> +and ioreadX(), iowriteX() and friends should not be necessary.
> +However, as the memory is not cache coherent, if access ever needs to
> +be protected by a spinlock then :c:func:`mmiowb()` must be used before
> +unlocking the lock. (See ACQUIRES VS I/O ACCESSES in
> +Documentation/memory-barriers.txt)