From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jesper Dangaard Brouer Subject: Re: [RFC] avoid indirect calls for DMA direct mappings Date: Thu, 6 Dec 2018 18:43:51 +0100 Message-ID: <20181206184351.4d9ece54@redhat.com> References: <20181206153720.10702-1-hch@lst.de> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20181206153720.10702-1-hch@lst.de> Sender: linux-kernel-owner@vger.kernel.org To: Christoph Hellwig Cc: iommu@lists.linux-foundation.org, Linus Torvalds , Tariq Toukan , Ilias Apalodimas , Toke =?UTF-8?B?SMO4aWxhbmQtSsO4?= =?UTF-8?B?cmdlbnNlbg==?= , Robin Murphy , linux-kernel@vger.kernel.org, brouer@redhat.com List-Id: iommu@lists.linux-foundation.org On Thu, 6 Dec 2018 07:37:19 -0800 Christoph Hellwig wrote: > Hi all, > > a while ago Jesper reported major performance regressions due to the > spectre v2 mitigations in his XDP forwarding workloads. A large part > of that is due to the DMA mapping API indirect calls. > > It turns out that the most common implementation of the DMA API is the > direct mapping case, and now that we have merged almost all duplicate > implementations of that into a single generic one is easily feasily to > direct calls for this fast path. > > This patch adds a check if we are using dma_direct_ops in each fast path > DMA operation, and just uses a direct call instead. For the XDP workload > this increases the number of packets per second from 7,438,283 to > 9,610,088, so it provides a very significant speedup. Full test report avail here: https://github.com/xdp-project/xdp-project/blob/master/areas/dma/dma01_test_hellwig_direct_dma.org > Note that the patch depends on a lot of work either queued up in the > DMA mapping tree, or still out on the list from review, so to actually > try the patch you probably want this git tree: > > > git://git.infradead.org/users/hch/misc.git dma-direct-calls > > Gitweb: > > http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dma-direct-calls > -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer