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 X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 47821C0044C for ; Mon, 29 Oct 2018 18:29:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1B8BC2080A for ; Mon, 29 Oct 2018 18:29:43 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1B8BC2080A Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729372AbeJ3DTb convert rfc822-to-8bit (ORCPT ); Mon, 29 Oct 2018 23:19:31 -0400 Received: from mx1.redhat.com ([209.132.183.28]:40138 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726852AbeJ3DTa (ORCPT ); Mon, 29 Oct 2018 23:19:30 -0400 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 1E2F8325DB6; Mon, 29 Oct 2018 18:29:41 +0000 (UTC) Received: from t450s.home (ovpn-116-100.phx2.redhat.com [10.3.116.100]) by smtp.corp.redhat.com (Postfix) with ESMTP id 9479D5FC0E; Mon, 29 Oct 2018 18:29:23 +0000 (UTC) Date: Mon, 29 Oct 2018 12:29:22 -0600 From: Alex Williamson To: Jason Wang Cc: Simon Guo , Eric Auger , qixuan.wu@linux.alibaba.com, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Peter Xu Subject: Re: Can VFIO pin only a specific region of guest mem when use pass through devices? Message-ID: <20181029122922.7b2a9b0c@t450s.home> In-Reply-To: References: <20181029024228.GA4279@simonLocalRHEL7.x64> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Mon, 29 Oct 2018 18:29:41 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 29 Oct 2018 17:14:46 +0800 Jason Wang wrote: > On 2018/10/29 上午10:42, Simon Guo wrote: > > Hi, > > > > I am using network device pass through mode with qemu x86(-device vfio-pci,host=0000:xx:yy.z) > > and “intel_iommu=on” in host kernel command line, and it shows the whole guest memory > > were pinned(vfio_pin_pages()), viewed by the “top” RES memory output. I understand it is due > > to device can DMA to any guest memory address and it cannot be swapped. > > > > However can we just pin a rang of address space allowed by iommu group of that device, > > instead of pin whole address space? I do notice some code like vtd_host_dma_iommu(). > > Maybe there is already some way to enable that? > > > > Sorry if I missed some basics. I googled some but no luck to find the answer yet. Please > > let me know if any discussion already raised on that. > > > > Any other suggestion will also be appreciated. For example, can we modify the guest network > > card driver to allocate only from a specific memory region(zone), and qemu advises guest > > kernel to only pin that memory region(zone) accordingly? > > > > Thanks, > > - Simon > > > One possible method is to enable IOMMU of VM. Right, making use of a virtual IOMMU in the VM is really the only way to bound the DMA to some subset of guest memory, but vIOMMU usage by the guest is optional on x86 and even if the guest does use it, it might enable passthrough mode, which puts you back at the problem that all guest memory is pinned with the additional problem that it might also be accounted for once per assigned device and may hit locked memory limits. Also, the DMA mapping and unmapping path with a vIOMMU is very slow, so performance of the device in the guest will be abysmal unless the use case is limited to very static mappings, such as userspace use within the guest for nested assignment or perhaps DPDK use cases. Modifying the guest to only use a portion of memory for DMA sounds like a quite intrusive option. There are certainly IOMMU models where the IOMMU provides a fixed IOVA range, but creating dynamic mappings within that range doesn't really solve anything given that it simply returns us to a vIOMMU with slow mapping. A window with a fixed identity mapping used as a DMA zone seems plausible, but again, also pretty intrusive to the guest, possibly also to the drivers. Host IOMMU page faulting can also help the pinned memory footprint, but of course requires hardware support and lots of new code paths, many of which are already being discussed for things like Scalable IOV and SVA. Thanks, Alex