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=-4.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 1E0E1C432BE for ; Fri, 27 Aug 2021 13:06:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EF6A560F92 for ; Fri, 27 Aug 2021 13:06:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S245113AbhH0NG5 (ORCPT ); Fri, 27 Aug 2021 09:06:57 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57914 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231271AbhH0NG4 (ORCPT ); Fri, 27 Aug 2021 09:06:56 -0400 Received: from mail-lf1-x12f.google.com (mail-lf1-x12f.google.com [IPv6:2a00:1450:4864:20::12f]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8BE5EC061757 for ; Fri, 27 Aug 2021 06:06:07 -0700 (PDT) Received: by mail-lf1-x12f.google.com with SMTP id b4so14160309lfo.13 for ; Fri, 27 Aug 2021 06:06:07 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=subject:to:cc:references:from:message-id:date:user-agent :mime-version:in-reply-to:content-transfer-encoding:content-language; bh=3tMnP0mNaBQdpdIRmVDF6sBZsrsvqJbngKw29spJzuE=; b=BMcKXNVwOo4A+nzmWzqT6hyQbkSuYzZctFkI6fRiKu+7QeUEtp3mpxq//lDi5W8Gmv k9zvi6QlayUrRChdH0xeKwg5b5+LJGsotpey+Cv6jZyLUSFLurbJecWXo+aau0WAGekl j+MTxSN07D9u0OouIlBQd0RcVPCVCWXSz+nTGUQXckakEWq8y2bZAUMNSEOYu+KxWZ4G Uulx9l+y7A9Quq3g1IA6ddBnieTFTq5v0aSCXAclZukqwEsluOcJzrkvjngeCAGZU2cf 8bd5I122nw3fzB209Diz/XI3Wf1eVSHakThzyOwwJQkoss1xO/k2i+CZMROPyk8hk3Go HPBw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-transfer-encoding :content-language; bh=3tMnP0mNaBQdpdIRmVDF6sBZsrsvqJbngKw29spJzuE=; b=M4sItcPel23fzykm7TGxEWgof7U+0K4gQmlU+K1Y4pQhurCX0D/ph0HI8tK7UvW1Wx 4Es3zp77Qlz7aBeaodF2xHkJ2x+7i5U5fKFJwl+DPbhle59XxdM9aiRMAAMtHBH/g65N iMsgBm+ZJmrzGxzrF5vgftSN/h+FUyO5RXCmwFSBqeAsH9Dj2/Tm+gy+9iXmJ/A2i6bL oTVEan0W7z9hb5iD56J8c7GJ9SJbThd84CKLsdFI8SCi6B29h9iGh63vYWB8hniJVVcv naEHdrw3m8PnQg2u3lDQzGYHRFZrjJHQDRbwpj8C5QvlCf1ndrsNJ8zXugPK4lyqgdoC M8dA== X-Gm-Message-State: AOAM5339IyhhfSr1YgFffFRDc4KW+JwnjcU0SG14AR/VXzOfXJxJXU6I LWRNvVhGQz5b26MqyLuWMzw= X-Google-Smtp-Source: ABdhPJx8F36Z+uhGUEPQDYWhnR/OQiJPAacK5hCGi+jpK9hJBZqj1cme6tSGeMg4ipbwPsU+VTZCVQ== X-Received: by 2002:a05:6512:1394:: with SMTP id p20mr6441639lfa.189.1630069565562; Fri, 27 Aug 2021 06:06:05 -0700 (PDT) Received: from [192.168.1.7] ([212.22.223.21]) by smtp.gmail.com with ESMTPSA id l13sm591052lfj.199.2021.08.27.06.06.04 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 27 Aug 2021 06:06:04 -0700 (PDT) Subject: Re: Clarification regarding updating "Xen hypervisor device tree bindings on Arm" To: robh+dt@kernel.org, devicetree@vger.kernel.org Cc: Julien Grall , Stefano Stabellini , Mark Rutland References: From: Oleksandr Message-ID: <8b311e33-89e5-87f3-63d2-54bbc2f8f8e7@gmail.com> Date: Fri, 27 Aug 2021 16:06:04 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: devicetree@vger.kernel.org Hello, all. Gentle reminder. On 07.08.21 01:57, Julien Grall wrote: > Hi Stefano, > > On 06/08/2021 23:26, Stefano Stabellini wrote: >> On Fri, 6 Aug 2021, Julien Grall wrote: >>> Hi Stefano, >>> >>> On 06/08/2021 21:15, Stefano Stabellini wrote: >>>> On Fri, 6 Aug 2021, Oleksandr Tyshchenko wrote: >>>>> Hello, all. >>>>> >>>>> I would like to clarify some bits regarding a possible update for >>>>> "Xen >>>>> device tree bindings for the guest" [1]. >>>>> >>>>> A bit of context: >>>>> We are considering extending "reg" property under the hypervisor >>>>> node and >>>>> we would like to avoid breaking backward compatibility. >>>>> So far, the "reg" was used to carry a single region for the grant >>>>> table >>>>> mapping only and it's size is quite small for the new improvement >>>>> we are currently working on. >>>>> >>>>> What we want to do is to extend the current region [reg: 0] and >>>>> add an >>>>> extra regions [reg: 1-N] to be used as a safe address space for any >>>>> Xen specific mappings. But, we need to be careful about running "new" >>>>> guests (with the improvement being built-in already) on "old" Xen >>>>> which is not aware of the extended regions, so we need the binding >>>>> to be >>>>> extended in a backward compatible way. In order to detect whether >>>>> we are running on top of the "new" Xen (and it provides us enough >>>>> space to >>>>> be used for improvement), we definitely need some sign to >>>>> indicate that. >>>>> >>>>> Could you please clarify, how do you expect the binding to be >>>>> changed in >>>>> the backward compatible way? >>>>> - by adding an extra compatible (as it is a change of the binding >>>>> technically) >>>>> - by just adding new property (xen,***) to indicate that "reg" >>>>> contains >>>>> enough space >>>>> - other option >>>>    The current description is: >>>> >>>> - reg: specifies the base physical address and size of a region in >>>>     memory where the grant table should be mapped to, using an >>>>     HYPERVISOR_memory_op hypercall [...] >>>> >>>> >>>> Although it says "a region" I think that adding multiple ranges >>>> would be >>>> fine and shouldn't break backward compatibility. >>>> >>>> In addition, the purpose of the region was described as "where the >>>> grant >>>> table should be mapped". In other words, it is a safe address range >>>> where the OS can map Xen special pages. >>>> >>>> Your proposal is to extend the region to be bigger to allow the OS to >>>> map more Xen special pages. I think it is a natural extension to the >>>> binding, which should be backward compatible. >>> >>> I agree that extending the reg (or even adding a second region) >>> should be fine >>> for older OS. >>> >>>> >>>> Rob, I am not sure what is commonly done in these cases. Maybe we just >>>> need an update to the description of the binding? I am also fine with >>>> adding a new compatible string if needed. >>> >>> So the trouble is how a newer Linux version knows that the region is >>> big >>> enough to deal with all the foreign/grant mapping? >>> >>> If you run on older Xen, then the region will only be 16MB. This >>> means the >>> Linux will have to fallback on stealing RAM as it is today. >>> >>> IOW, XSA-300 will still be a thing. On newer Xen (or toolstack), we >>> ideally >>> want the OS to not fallback on stealing RAM (and close XSA-300). >>> This is where >>> we need a way to advertise it. >>> >>> The question here is whether we want to use a property or a >>> compatible for >>> this. >>> >>> I am leaning towards the latter because this is an extension of the >>> bindings. >>> However, I wasn't entirely whether this was a normal way to do it. May I please ask for the clarification how to properly advertise that we have extended region? By new compatible or property? >> >> Although I think it would be OK to have a new compatible string, am I >> not sure we need it. > > Let's assume we don't add a new compatible string, property... How do > would you prevent the following two issues? >   1) XSA-300: A frontend can DoS the backend >   2) Existing Xen expects the grant-table to be mapped at the exact > same place. > > 2# could potentially be solved by reserved the first range for the > grant table. For 1#, I think we need a compatible string (or property). > > What else do you have in mind? > > FAOD, relying on the region to always be big enough would not be an > acceptable solution to me :). A frontend may find a new way for a > frontend to exhaust the region (*hint* virtio *hint*). > >> >> In any case, we'll have to be able to recognize and handle the case >> where we run out of the space in the provided region. If the region is >> too small (16MB) then it just means we'll run out of space immediately >> after mapping the grant table. Then, we'll have to use other techniques. > > Right, one of the other techniques is likely to steal RAM page. Which > means that a frontend could potentially DoS the backend. This will be > a lot easier to trigger with virtio as the DM tends to cache the > mappings. > > So I think we ought to prevent stealing the RAM if a new kernel is > running on a new Xen. > >> >> Or perhaps you think that if we had a new compatible string to say "Xen >> binding with a larger region" then we could get away with a simpler >> implementation in Linux, one that doesn't handle the case where we run >> out of space in the region? If that was the case, then I agree that it >> would be worthwhile adding a new compatible. > > We will need to keep the code to steal RAM for the forseeable future > because a newer Linux may run on an older Xen setup. So simplicity is > not the reason here. > > I have provided the reason above. -- Regards, Oleksandr Tyshchenko