From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Gunthorpe Subject: Re: [RFC PATCH 00/28] Removing struct page from P2PDMA Date: Mon, 24 Jun 2019 10:55:50 -0300 Message-ID: <20190624135550.GB8268@ziepe.ca> References: <20190620161240.22738-1-logang@deltatee.com> <20190620193353.GF19891@ziepe.ca> <20190624073126.GB3954@lst.de> <20190624134641.GA8268@ziepe.ca> <20190624135024.GA11248@lst.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20190624135024.GA11248@lst.de> Sender: linux-kernel-owner@vger.kernel.org To: Christoph Hellwig Cc: Dan Williams , Logan Gunthorpe , Linux Kernel Mailing List , linux-block@vger.kernel.org, linux-nvme@lists.infradead.org, linux-pci@vger.kernel.org, linux-rdma , Jens Axboe , Bjorn Helgaas , Sagi Grimberg , Keith Busch , Stephen Bates List-Id: linux-rdma@vger.kernel.org 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