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=-6.3 required=3.0 tests=BAYES_00, 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 393A0C433E0 for ; Tue, 28 Jul 2020 19:55:01 +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 106B92075A for ; Tue, 28 Jul 2020 19:55:00 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 106B92075A 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 1k0VgN-0000yl-R9; Tue, 28 Jul 2020 19:54:39 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1k0VgM-0000yg-Is for xen-devel@lists.xenproject.org; Tue, 28 Jul 2020 19:54:38 +0000 X-Inumbo-ID: 2a6babeb-d10c-11ea-8bce-bc764e2007e4 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id 2a6babeb-d10c-11ea-8bce-bc764e2007e4; Tue, 28 Jul 2020 19:54:37 +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 B9396AD6F; Tue, 28 Jul 2020 19:54:47 +0000 (UTC) Subject: Re: [PATCH v2] xen/arm: Convert runstate address during hypcall To: Bertrand Marquis References: <4647a019c7b42d40d3c2f5b0a3685954bea7f982.1595948219.git.bertrand.marquis@arm.com> From: Jan Beulich Message-ID: <8d2d7f03-450c-d50c-630b-8608c6d42bb9@suse.com> Date: Tue, 28 Jul 2020 21:54:35 +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: <4647a019c7b42d40d3c2f5b0a3685954bea7f982.1595948219.git.bertrand.marquis@arm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit 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: Stefano Stabellini , Julien Grall , Wei Liu , Andrew Cooper , Ian Jackson , George Dunlap , xen-devel@lists.xenproject.org, nd@arm.com, Volodymyr Babchuk , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" 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). > --- a/xen/arch/x86/domain.c > +++ b/xen/arch/x86/domain.c > @@ -1642,6 +1642,30 @@ void paravirt_ctxt_switch_to(struct vcpu *v) > wrmsr_tsc_aux(v->arch.msrs->tsc_aux); > } > > +int arch_vcpu_setup_runstate(struct vcpu *v, > + struct vcpu_register_runstate_memory_area area) > +{ > + struct vcpu_runstate_info runstate; > + > + runstate_guest(v) = area.addr.h; > + > + if ( v == current ) > + { > + __copy_to_guest(runstate_guest(v), &v->runstate, 1); > + } Pointless braces (and I think there are more instances). > + else > + { > + vcpu_runstate_get(v, &runstate); > + __copy_to_guest(runstate_guest(v), &runstate, 1); > + } > + return 0; Missing blank line before main "return". Jan