linux-kernel - Re: [RFC PATCH 00/28] Removing struct page from P2PDMA

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Message-ID: <7210ba39-c923-79ca-57bb-7cf9afe21d54@deltatee.com>
Date:   Mon, 24 Jun 2019 10:53:38 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgg@...pe.ca>, Christoph Hellwig <hch@....de>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-block@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-pci@...r.kernel.org, linux-rdma <linux-rdma@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Keith Busch <kbusch@...nel.org>,
        Stephen Bates <sbates@...thlin.com>
Subject: Re: [RFC PATCH 00/28] Removing struct page from P2PDMA



On 2019-06-24 7:55 a.m., Jason Gunthorpe wrote:
> On Mon, Jun 24, 2019 at 03:50:24PM +0200, Christoph Hellwig wrote:
>> On Mon, Jun 24, 2019 at 10:46:41AM -0300, Jason Gunthorpe wrote:
>>> BTW, it is not just offset right? It is possible that the IOMMU can
>>> generate unique dma_addr_t values for each device?? Simple offset is
>>> just something we saw in certain embedded cases, IIRC.
>>
>> Yes, it could.  If we are trying to do P2P between two devices on
>> different root ports and with the IOMMU enabled we'll generate
>> a new bus address for the BAR on the other side dynamically everytime
>> we map.
> 
> Even with the same root port if ACS is turned on could behave like this.

Yup.

> It is only a very narrow case where you can take shortcuts with
> dma_addr_t, and I don't think shortcuts like are are appropriate for
> the mainline kernel..

I don't think it's that narrow and it opens up a lot of avenues for
system design that people are wanting to go. If your high speed data
path can avoid the root complex and CPU, you can design a system which a
much smaller CPU and fewer lanes directed at the CPU.

Logan