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Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.9.0 MIME-Version: 1.0 In-Reply-To: <37fa46c7-2c24-1808-16e9-e543f4601279@nvidia.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-CA Content-Transfer-Encoding: 7bit X-SA-Exim-Connect-IP: 172.16.1.162 X-SA-Exim-Rcpt-To: robin.murphy@arm.com, ira.weiny@intel.com, helgaas@kernel.org, jianxin.xiong@intel.com, dave.hansen@linux.intel.com, jason@jlekstrand.net, dave.b.minturn@intel.com, andrzej.jakowski@intel.com, daniel.vetter@ffwll.ch, willy@infradead.org, ddutile@redhat.com, christian.koenig@amd.com, jgg@ziepe.ca, dan.j.williams@intel.com, hch@lst.de, 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, jhubbard@nvidia.com X-SA-Exim-Mail-From: logang@deltatee.com Subject: Re: [PATCH 09/16] dma-direct: Support PCI P2PDMA pages in dma-direct map_sg 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-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: D1F43500153B Authentication-Results: imf01.hostedemail.com; dkim=pass header.d=deltatee.com header.s=20200525 header.b=A4opownL; dmarc=none; spf=pass (imf01.hostedemail.com: domain of logang@deltatee.com designates 204.191.154.188 as permitted sender) smtp.mailfrom=logang@deltatee.com X-Stat-Signature: nreppfaaci86ith1qimh4sdkg4nuhy4j Received-SPF: none (deltatee.com>: No applicable sender policy available) receiver=imf01; identity=mailfrom; envelope-from=""; helo=ale.deltatee.com; client-ip=204.191.154.188 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1620061459-207763 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Oops missed a comment: On 2021-05-02 5:28 p.m., John Hubbard wrote: >> int dma_direct_map_sg(struct device *dev, struct scatterlist *sgl, int nents, >> enum dma_data_direction dir, unsigned long attrs) >> { >> - int i; >> + struct pci_p2pdma_map_state p2pdma_state = {}; > > Is it worth putting this stuff on the stack--is there a noticeable > performance improvement from caching the state? Because if it's > invisible, then simplicity is better. I suspect you're right, and that > it *is* worth it, but it's good to know for real. I haven't measured it (it would be hard to measure), but I think it's fairly clear here. Without the state, xa_load() would need to be called on *every* page in an SGL that maps only P2PDMA memory from one device. With the state, it only needs to be called once. xa_load() is cheap, but it is not that cheap. There's essentially the same optimization in get_user_pages for ZONE_DEVICE pages. So, if it is necessary there, it should be necessary here. Logan