Subject: Re: [PATCH 0/3] iopmem : A block device for PCIe memory
To: Dan Williams <dan.j.williams@intel.com>,
        Stephen Bates <sbates@raithlin.com>
References: <1476826937-20665-1-git-send-email-sbates@raithlin.com>
 <CAPcyv4gJ_c-6s2BUjsu6okR1EF53R+KNuXnOc5jv0fuwJaa3cQ@mail.gmail.com>
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        Linux MM <linux-mm@kvack.org>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
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From: Haggai Eran <haggaie@mellanox.com>
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Date: Wed, 26 Oct 2016 11:24:51 +0300
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On 10/19/2016 6:51 AM, Dan Williams wrote:
> On Tue, Oct 18, 2016 at 2:42 PM, Stephen Bates <sbates@raithlin.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