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=-8.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 1C3AEC4361A for ; Thu, 3 Dec 2020 16:04:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BFC57207AA for ; Thu, 3 Dec 2020 16:04:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389256AbgLCQDr (ORCPT ); Thu, 3 Dec 2020 11:03:47 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:46736 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387964AbgLCQDq (ORCPT ); Thu, 3 Dec 2020 11:03:46 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1607011339; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=gwFT/at38qnWnV6ALWueYTdB/e9xeSjOOzsCIelui5c=; b=J077cbwkt0N+gbVQ/zN3V4NwntwIwk0njyxbsah2feICeNHidYidvM9RfjktwwbEvUb31Y wM8tHQDYKvmpE+pYfzIyX3Cy+h6q5YXcV+jkF1be21OkHgvCCR726NXxiO6J5mzOTD+elX l6rme2VgZXvxXMqMJNiV+zfdnUxc8/g= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-495-uSXkKsj0OB6QOXi442mkng-1; Thu, 03 Dec 2020 11:02:17 -0500 X-MC-Unique: uSXkKsj0OB6QOXi442mkng-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 51A461086607; Thu, 3 Dec 2020 16:01:35 +0000 (UTC) Received: from [10.36.113.250] (ovpn-113-250.ams2.redhat.com [10.36.113.250]) by smtp.corp.redhat.com (Postfix) with ESMTP id 972265D6BA; Thu, 3 Dec 2020 16:01:33 +0000 (UTC) Subject: Re: [PATCH] vfio iommu type1: Bypass the vma permission check in vfio_pin_pages_remote() To: Peter Xu , Stefan Hajnoczi Cc: Justin He , Alex Williamson , Cornelia Huck , "kvm@vger.kernel.org" , "linux-kernel@vger.kernel.org" References: <20201119142737.17574-1-justin.he@arm.com> <20201124181228.GA276043@xz-x1> <20201125155711.GA6489@xz-x1> <20201202143356.GK655829@stefanha-x1.localdomain> <20201202154511.GI3277@xz-x1> <20201203112002.GE689053@stefanha-x1.localdomain> <20201203154322.GH108496@xz-x1> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <6a33e908-17ff-7a26-7341-4bcf7bbefe28@redhat.com> Date: Thu, 3 Dec 2020 17:01:32 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.4.0 MIME-Version: 1.0 In-Reply-To: <20201203154322.GH108496@xz-x1> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 03.12.20 16:43, Peter Xu wrote: > On Thu, Dec 03, 2020 at 11:20:02AM +0000, Stefan Hajnoczi wrote: >> On Wed, Dec 02, 2020 at 10:45:11AM -0500, Peter Xu wrote: >>> On Wed, Dec 02, 2020 at 02:33:56PM +0000, Stefan Hajnoczi wrote: >>>> On Wed, Nov 25, 2020 at 10:57:11AM -0500, Peter Xu wrote: >>>>> On Wed, Nov 25, 2020 at 01:05:25AM +0000, Justin He wrote: >>>>>>> I'd appreciate if you could explain why vfio needs to dma map some >>>>>>> PROT_NONE >>>>>> >>>>>> Virtiofs will map a PROT_NONE cache window region firstly, then remap the sub >>>>>> region of that cache window with read or write permission. I guess this might >>>>>> be an security concern. Just CC virtiofs expert Stefan to answer it more accurately. >>>>> >>>>> Yep. Since my previous sentence was cut off, I'll rephrase: I was thinking >>>>> whether qemu can do vfio maps only until it remaps the PROT_NONE regions into >>>>> PROT_READ|PROT_WRITE ones, rather than trying to map dma pages upon PROT_NONE. >>>> >>>> Userspace processes sometimes use PROT_NONE to reserve virtual address >>>> space. That way future mmap(NULL, ...) calls will not accidentally >>>> allocate an address from the reserved range. >>>> >>>> virtio-fs needs to do this because the DAX window mappings change at >>>> runtime. Initially the entire DAX window is just reserved using >>>> PROT_NONE. When it's time to mmap a portion of a file into the DAX >>>> window an mmap(fixed_addr, ...) call will be made. >>> >>> Yes I can understand the rational on why the region is reserved. However IMHO >>> the real question is why such reservation behavior should affect qemu memory >>> layout, and even further to VFIO mappings. >>> >>> Note that PROT_NONE should likely mean that there's no backing page at all in >>> this case. Since vfio will pin all the pages before mapping the DMAs, it also >>> means that it's at least inefficient, because when we try to map all the >>> PROT_NONE pages we'll try to fault in every single page of it, even if they may >>> not ever be used. >>> >>> So I still think this patch is not doing the right thing. Instead we should >>> somehow teach qemu that the virtiofs memory region should only be the size of >>> enabled regions (with PROT_READ|PROT_WRITE), rather than the whole reserved >>> PROT_NONE region. >> >> virtio-fs was not implemented with IOMMUs in mind. The idea is just to >> install a kvm.ko memory region that exposes the DAX window. >> >> Perhaps we need to treat the DAX window like an IOMMU? That way the >> virtio-fs code can send map/unmap notifications and hw/vfio/ can >> propagate them to the host kernel. > > Sounds right. One more thing to mention is that we may need to avoid tearing > down the whole old DMA region when resizing the PROT_READ|PROT_WRITE region > into e.g. a bigger one to cover some of the previusly PROT_NONE part, as long > as if the before-resizing region is still possible to be accessed from any > hardware. It smells like something David is working with virtio-mem, not sure > whether there's any common infrastructure that could be shared. "somehow teach qemu that the virtiofs memory region should only be the size of enabled regions" - for virtio-mem, I'm working on resizeable RAM blocks/RAM memory regions. Fairly complicated, that's why I deferred upstreaming it and still need to implement plenty of special cases for atomic resizes (e.g., vhost-user). But it's only one part of the puzzle for virtio-fs. AFAIU, it's not only about resizing the region for virtio-fs - we can have PROT_NONE holes anywhere inside there. In vfio, you cannot shrink mappings atomically. Growing works, but requires additional mappings (-> bad). So assume you mapped a file with size X and want to resize it. You first have to unmap + remap with the new size. This is not atomic, thus problematic. For virtio-mem, we have a fix block size and can map/unmap in that granularity whenever we populate/discard memory within the device-managed region. I don't think that applies for files in case of virtio-fs. The real question is: do we even *need* DMA from vfio devices to virtio-fs regions? If not (do guests rely on it? what does the spec state?), just don't care about vfio at all and don't map anything. But maybe I am missing something important Q1: Is the virtio-fs region mapped into system address space, so we have to map everything without a vIOMMU? Q2: Is DMA from vfio devices to virtio-fs regions a valid use case? -- Thanks, David / dhildenb