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Thu, 10 Dec 2020 09:44:09 -0700 To: Dan Williams References: <20201106170036.18713-1-logang@deltatee.com> <20201106170036.18713-5-logang@deltatee.com> <20201109091258.GB28918@lst.de> <4e336c7e-207b-31fa-806e-c4e8028524a5@deltatee.com> From: Logan Gunthorpe Message-ID: <7b08e6e9-eca4-0070-8444-5eb00965a0ad@deltatee.com> Date: Thu, 10 Dec 2020 09:44:03 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.1 MIME-Version: 1.0 In-Reply-To: Content-Language: en-US X-SA-Exim-Connect-IP: 24.64.145.4 X-SA-Exim-Rcpt-To: daniel.vetter@ffwll.ch, willy@infradead.org, ddutile@redhat.com, jhubbard@nvidia.com, christian.koenig@amd.com, jgg@ziepe.ca, sbates@raithlin.com, iommu@lists.linux-foundation.org, linux-mm@kvack.org, linux-pci@vger.kernel.org, linux-block@vger.kernel.org, linux-nvme@lists.infradead.org, linux-kernel@vger.kernel.org, hch@lst.de, dan.j.williams@intel.com X-SA-Exim-Mail-From: logang@deltatee.com Subject: Re: [RFC PATCH 04/15] lib/scatterlist: Add flag for indicating P2PDMA segments in an SGL X-SA-Exim-Version: 4.2.1 (built Wed, 08 May 2019 21:11:16 +0000) X-SA-Exim-Scanned: Yes (on ale.deltatee.com) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201210_114418_259803_351BAA3B X-CRM114-Status: GOOD ( 22.76 ) X-BeenThere: linux-nvme@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Jason Gunthorpe , Linux PCI , Daniel Vetter , Linux Kernel Mailing List , linux-nvme@lists.infradead.org, Stephen Bates , linux-block@vger.kernel.org, Linux MM , "open list:DMA MAPPING HELPERS" , Don Dutile , Matthew Wilcox , John Hubbard , =?UTF-8?Q?Christian_K=c3=b6nig?= , Christoph Hellwig Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "Linux-nvme" Errors-To: linux-nvme-bounces+linux-nvme=archiver.kernel.org@lists.infradead.org On 2020-12-09 9:04 p.m., Dan Williams wrote: > On Wed, Dec 9, 2020 at 6:07 PM Logan Gunthorpe wrote: >> >> >> >> On 2020-12-09 6:22 p.m., Dan Williams wrote: >>> On Mon, Nov 9, 2020 at 8:47 AM Logan Gunthorpe wrote: >>>> >>>> >>>> >>>> On 2020-11-09 2:12 a.m., Christoph Hellwig wrote: >>>>> On Fri, Nov 06, 2020 at 10:00:25AM -0700, Logan Gunthorpe wrote: >>>>>> We make use of the top bit of the dma_length to indicate a P2PDMA >>>>>> segment. >>>>> >>>>> I don't think "we" can. There is nothing limiting the size of a SGL >>>>> segment. >>>> >>>> Yes, I expected this would be the unacceptable part. Any alternative ideas? >>> >>> Why is the SG_P2PDMA_FLAG needed as compared to checking the SGL >>> segment-pages for is_pci_p2pdma_page()? >> >> Because the DMA and page segments in the SGL aren't necessarily aligned... >> >> The IOMMU implementations can coalesce multiple pages into fewer DMA >> address ranges, so the page pointed to by sg->page_link may not be the >> one that corresponds to the address in sg->dma_address for a given segment. >> >> If that makes sense -- it's not the easiest thing to explain. > > It does... > > Did someone already grab, or did you already consider the 3rd > available bit in page_link? AFAICS only SG_CHAIN and SG_END are > reserved. However, if you have a CONFIG_64BIT dependency for > user-directed p2pdma that would seem to allow SG_P2PDMA_FLAG to be > (0x4) in page_link. Hmm, I half considered that, but I had came to the conclusion that given the mis-alignment I shouldn't be using the page side of the SGL. However, reconsidering now, that might actually be a reasonable option. However, the CONFIG_64BIT dependency would have to be on all P2PDMA, because we'd need to replace pci_p2pdma_map_sg() in all cases. I'm not sure if this would be a restriction people care about. Thanks, Logan _______________________________________________ Linux-nvme mailing list Linux-nvme@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-nvme