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

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Message-ID: <20190624135550.GB8268@ziepe.ca>
Date:   Mon, 24 Jun 2019 10:55:50 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Christoph Hellwig <hch@....de>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Logan Gunthorpe <logang@...tatee.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 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.

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..

Jason