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Tue, 4 May 2021 00:01:32 +0000 Received: from [10.2.50.162] (172.20.145.6) by HQMAIL107.nvidia.com (172.20.187.13) with Microsoft SMTP Server (TLS) id 15.0.1497.2; Tue, 4 May 2021 00:01:30 +0000 Subject: Re: [PATCH 09/16] dma-direct: Support PCI P2PDMA pages in dma-direct map_sg To: Logan Gunthorpe , , , , , , CC: Stephen Bates , Christoph Hellwig , Dan Williams , Jason Gunthorpe , =?UTF-8?Q?Christian_K=c3=b6nig?= , Don Dutile , Matthew Wilcox , Daniel Vetter , Jakowski Andrzej , Minturn Dave B , Jason Ekstrand , Dave Hansen , Xiong Jianxin , Bjorn Helgaas , Ira Weiny , Robin Murphy References: <20210408170123.8788-1-logang@deltatee.com> <20210408170123.8788-10-logang@deltatee.com> <37fa46c7-2c24-1808-16e9-e543f4601279@nvidia.com> <8de928ab-2842-dac9-07ad-a098124f791f@deltatee.com> From: John Hubbard Message-ID: <2a08b685-53c1-3ee4-0ce1-315554d63685@nvidia.com> Date: Mon, 3 May 2021 17:01:29 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.10.0 MIME-Version: 1.0 In-Reply-To: <8de928ab-2842-dac9-07ad-a098124f791f@deltatee.com> Content-Type: text/plain; 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X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 04 May 2021 00:01:32.1844 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 0813dd7d-b8d3-4a4a-ab1e-08d90e8fc688 X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.112.34];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: BN8NAM11FT068.eop-nam11.prod.protection.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM6PR12MB4419 Authentication-Results: imf13.hostedemail.com; dkim=pass header.d=Nvidia.com header.s=selector2 header.b=jJ3JtgOs; dmarc=pass (policy=none) header.from=nvidia.com; spf=none (imf13.hostedemail.com: domain of jhubbard@nvidia.com has no SPF policy when checking 40.107.212.67) smtp.mailfrom=jhubbard@nvidia.com X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 4D39BE00012D X-Stat-Signature: zhiq41fbqtthcpjix6erbu7u3thnoyum Received-SPF: none (nvidia.com>: No applicable sender policy available) receiver=imf13; identity=mailfrom; envelope-from=""; helo=NAM02-BN1-obe.outbound.protection.outlook.com; client-ip=40.107.212.67 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1620086484-188288 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: On 5/3/21 10:04 AM, Logan Gunthorpe wrote: > 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. OK, thanks for spelling it out for me. :) > > 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. > Right, that's a pretty solid example. thanks, -- John Hubbard NVIDIA