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=-6.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,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 AA482C433DF for ; Fri, 31 Jul 2020 06:39:38 +0000 (UTC) Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 83528207F5 for ; Fri, 31 Jul 2020 06:39:38 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 83528207F5 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1k1OhM-0002tq-B8; Fri, 31 Jul 2020 06:39:20 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1k1OhK-0002tl-Sm for xen-devel@lists.xenproject.org; Fri, 31 Jul 2020 06:39:18 +0000 X-Inumbo-ID: 8e7eddda-d2f8-11ea-8e0f-bc764e2007e4 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id 8e7eddda-d2f8-11ea-8e0f-bc764e2007e4; Fri, 31 Jul 2020 06:39:17 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id C3FEAAC5E; Fri, 31 Jul 2020 06:39:29 +0000 (UTC) Subject: Re: [PATCH v2] xen/arm: Convert runstate address during hypcall To: Stefano Stabellini References: <4647a019c7b42d40d3c2f5b0a3685954bea7f982.1595948219.git.bertrand.marquis@arm.com> <8d2d7f03-450c-d50c-630b-8608c6d42bb9@suse.com> <1b046f2c-05c8-9276-a91e-fd55ec098bed@suse.com> From: Jan Beulich Message-ID: <1a8bbcc7-9d0c-9669-db7b-e837af279027@suse.com> Date: Fri, 31 Jul 2020 08:39:16 +0200 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Cc: Julien Grall , Wei Liu , Andrew Cooper , Ian Jackson , George Dunlap , Bertrand Marquis , Xen-devel , nd , Volodymyr Babchuk , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" On 30.07.2020 03:30, Stefano Stabellini wrote: > On Wed, 29 Jul 2020, Jan Beulich wrote: >> On 29.07.2020 09:08, Bertrand Marquis wrote: >>>> On 28 Jul 2020, at 21:54, Jan Beulich wrote: >>>> On 28.07.2020 17:52, Bertrand Marquis wrote: >>>>> At the moment on Arm, a Linux guest running with KTPI enabled will >>>>> cause the following error when a context switch happens in user mode: >>>>> (XEN) p2m.c:1890: d1v0: Failed to walk page-table va 0xffffff837ebe0cd0 >>>>> The error is caused by the virtual address for the runstate area >>>>> registered by the guest only being accessible when the guest is running >>>>> in kernel space when KPTI is enabled. >>>>> To solve this issue, this patch is doing the translation from virtual >>>>> address to physical address during the hypercall and mapping the >>>>> required pages using vmap. This is removing the conversion from virtual >>>>> to physical address during the context switch which is solving the >>>>> problem with KPTI. >>>>> This is done only on arm architecture, the behaviour on x86 is not >>>>> modified by this patch and the address conversion is done as before >>>>> during each context switch. >>>>> This is introducing several limitations in comparison to the previous >>>>> behaviour (on arm only): >>>>> - if the guest is remapping the area at a different physical address Xen >>>>> will continue to update the area at the previous physical address. As >>>>> the area is in kernel space and usually defined as a global variable >>>>> this >>>>> is something which is believed not to happen. If this is required by a >>>>> guest, it will have to call the hypercall with the new area (even if it >>>>> is at the same virtual address). >>>>> - the area needs to be mapped during the hypercall. For the same reasons >>>>> as for the previous case, even if the area is registered for a different >>>>> vcpu. It is believed that registering an area using a virtual address >>>>> unmapped is not something done. >>>> >>>> Beside me thinking that an in-use and stable ABI can't be changed like >>>> this, no matter what is "believed" kernel code may or may not do, I >>>> also don't think having arch-es diverge in behavior here is a good >>>> idea. Use of commonly available interfaces shouldn't lead to head >>>> aches or surprises when porting code from one arch to another. I'm >>>> pretty sure it was suggested before: Why don't you simply introduce >>>> a physical address based hypercall (and then also on x86 at the same >>>> time, keeping functional parity)? I even seem to recall giving a >>>> suggestion how to fit this into a future "physical addresses only" >>>> model, as long as we can settle on the basic principles of that >>>> conversion path that we want to go sooner or later anyway (as I >>>> understand). >>> >>> I fully agree with the “physical address only” model and i think it must be >>> done. Introducing a new hypercall taking a physical address as parameter >>> is the long term solution (and I would even volunteer to do it in a new >>> patchset). >>> But this would not solve the issue here unless linux is modified. >>> So I do see this patch as a “bug fix”. >> >> Well, it is sort of implied by my previous reply that we won't get away >> without an OS side change here. The prereq to get away without would be >> that it is okay to change the behavior of a hypercall like you do, and >> that it is okay to make the behavior diverge between arch-es. I think >> I've made pretty clear that I don't think either is really an option. > > This is a difficult problem to solve and the current situation honestly > sucks: there is no way to solve the problem without making compromises. > > The new hypercall is good-to-have in any case (it is a better interface) > but it is not a full solution. If we introduce a new hypercall we fix > new guests but don't fix existing guests. If we change Linux in any way, > we are still going to have problems with all already-released kernel > binaries. Leaving the issue unfixed is not an option either because the > problem can't be ignored. We're fixing other issues without breaking the ABI. Where's the problem of backporting the kernel side change (which I anticipate to not be overly involved)? If the plan remains to be to make an ABI breaking change, then I think this will need an explicit vote. Jan