linux-kernel - Re: [PATCH 0/3] iopmem : A block device for PCIe memory

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Date:   Wed, 26 Oct 2016 11:24:51 +0300
From:   Haggai Eran <haggaie@...lanox.com>
To:     Dan Williams <dan.j.williams@...el.com>,
        Stephen Bates <sbates@...thlin.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        <linux-rdma@...r.kernel.org>, <linux-block@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        "Matthew Wilcox" <willy@...ux.intel.com>,
        <jgunthorpe@...idianresearch.com>,
        "Christoph Hellwig" <hch@...radead.org>, Jens Axboe <axboe@...com>,
        Jonathan Corbet <corbet@....net>, <jim.macdonald@...rspin.com>,
        <sbates@...thin.com>, "Logan Gunthorpe" <logang@...tatee.com>,
        David Woodhouse <dwmw2@...radead.org>,
        "Raj, Ashok" <ashok.raj@...el.com>
Subject: Re: [PATCH 0/3] iopmem : A block device for PCIe memory

On 10/19/2016 6:51 AM, Dan Williams wrote:
> On Tue, Oct 18, 2016 at 2:42 PM, Stephen Bates <sbates@...thlin.com> wrote:
>> 1. Address Translation. Suggestions have been made that in certain
>> architectures and topologies the dma_addr_t passed to the DMA master
>> in a peer-2-peer transfer will not correctly route to the IO memory
>> intended. However in our testing to date we have not seen this to be
>> an issue, even in systems with IOMMUs and PCIe switches. It is our
>> understanding that an IOMMU only maps system memory and would not
>> interfere with device memory regions.
I'm not sure that's the case. I think it works because with ZONE_DEVICE,
the iommu driver will simply treat a dma_map_page call as any other PFN,
and create a mapping as it does for any memory page.

>> (It certainly has no opportunity
>> to do so if the transfer gets routed through a switch).
It can still go through the IOMMU if you enable ACS upstream forwarding.

> There may still be platforms where peer-to-peer cycles are routed up
> through the root bridge and then back down to target device, but we
> can address that when / if it happens.
I agree.

> I wonder if we could (ab)use a
> software-defined 'pasid' as the requester id for a peer-to-peer
> mapping that needs address translation.
Why would you need that? Isn't it enough to map the peer-to-peer
addresses correctly in the iommu driver?

Haggai