From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 154AACE7A81 for ; Mon, 25 Sep 2023 11:15:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230438AbjIYLQB (ORCPT ); Mon, 25 Sep 2023 07:16:01 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41270 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229556AbjIYLP7 (ORCPT ); Mon, 25 Sep 2023 07:15:59 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id ED37EC6 for ; Mon, 25 Sep 2023 04:15:51 -0700 (PDT) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 67847DA7; Mon, 25 Sep 2023 04:16:29 -0700 (PDT) Received: from [10.57.0.188] (unknown [10.57.0.188]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5E2BC3F59C; Mon, 25 Sep 2023 04:15:50 -0700 (PDT) Message-ID: <92cd8f47-054c-938a-0dcb-778ed42805ed@arm.com> Date: Mon, 25 Sep 2023 12:15:43 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:102.0) Gecko/20100101 Thunderbird/102.15.1 Subject: Re: dma_map_resource() has a bad performance in pcie peer to peer transactions when iommu enabled in Linux Content-Language: en-GB To: Kelly Devilliv , "joro@8bytes.org" , "will@kernel.org" Cc: "iommu@lists.linux.dev" , "linux-kernel@vger.kernel.org" References: From: Robin Murphy In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2023-09-25 04:59, Kelly Devilliv wrote: > Dear all, > > I am working on an ARM-V8 server with two gpu cards on it. Recently, I need to test pcie peer to peer communication between the two gpu cards, but the throughput is only 4GB/s. > > After I explored the gpu's kernel mode driver, I found it was using the dma_map_resource() API to map the peer device's MMIO space. The arm iommu driver then will hardcode a 'IOMMU_MMIO' prot in the later dma map: > > static dma_addr_t iommu_dma_map_resource(struct device *dev, phys_addr_t phys, > size_t size, enum dma_data_direction dir, unsigned long attrs) > { > return __iommu_dma_map(dev, phys, size, > dma_info_to_prot(dir, false, attrs) | IOMMU_MMIO, > dma_get_mask(dev)); > } > > And that will finally set the 'ARM_LPAE_PTE_MEMATTR_DEV' attribute in PTE, which may have a negative impact on the performance of the pcie peer to peer transactions. > > /* > * Note that this logic is structured to accommodate Mali LPAE > * having stage-1-like attributes but stage-2-like permissions. > */ > if (data->iop.fmt == ARM_64_LPAE_S2 || > data->iop.fmt == ARM_32_LPAE_S2) { > if (prot & IOMMU_MMIO) > pte |= ARM_LPAE_PTE_MEMATTR_DEV; > else if (prot & IOMMU_CACHE) > pte |= ARM_LPAE_PTE_MEMATTR_OIWB; > else > pte |= ARM_LPAE_PTE_MEMATTR_NC; > } else { > if (prot & IOMMU_MMIO) > pte |= (ARM_LPAE_MAIR_ATTR_IDX_DEV > << ARM_LPAE_PTE_ATTRINDX_SHIFT); > else if (prot & IOMMU_CACHE) > pte |= (ARM_LPAE_MAIR_ATTR_IDX_CACHE > << ARM_LPAE_PTE_ATTRINDX_SHIFT); > } > > I tried to remove the 'IOMMU_MMIO' prot in the dma_map_resource() API and re-compile the linux kernel, the throughput then can be up to 28GB/s. > > Is there an elegant way to solve this issue without modifying the linux kernel? e.g., a substitution of dma_map_resource() API? Not really. Other use-cases for dma_map_resource() include DMA offload engines accessing FIFO registers, where allowing reordering, write-gathering, etc. would be a terrible idea. Thus it needs to assume a "safe" MMIO memory type, which on Arm means Device-nGnRE. However, the "proper" PCI peer-to-peer support under CONFIG_PCI_P2PDMA ended up moving away from the dma_map_resource() approach anyway, and allows this kind of device memory to be treated more like regular memory (via ZONE_DEVICE) rather than arbitrary MMIO resources, so your best bet would be to get the GPU driver converted over to using that. Thanks, Robin. > > Thank you! > > Platform info: > Linux kernel version: 5.10 > PCIE GEN4 x16 > > Sincerely, > Kelly >