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=-1.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 CA851C10F13 for ; Mon, 8 Apr 2019 17:32:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 934F92148E for ; Mon, 8 Apr 2019 17:32:04 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=oracle.com header.i=@oracle.com header.b="Ci9NvxSe" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728548AbfDHRcD (ORCPT ); Mon, 8 Apr 2019 13:32:03 -0400 Received: from userp2130.oracle.com ([156.151.31.86]:37150 "EHLO userp2130.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726025AbfDHRcD (ORCPT ); Mon, 8 Apr 2019 13:32:03 -0400 Received: from pps.filterd (userp2130.oracle.com [127.0.0.1]) by userp2130.oracle.com (8.16.0.27/8.16.0.27) with SMTP id x38HSbF0075374; Mon, 8 Apr 2019 17:31:43 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=oracle.com; h=subject : to : cc : references : from : message-id : date : mime-version : in-reply-to : content-type : content-transfer-encoding; s=corp-2018-07-02; bh=TC6Rr4dhZ4pLAIm0OFKokXoDpFtW+09az8JdggeU5s4=; b=Ci9NvxSeLqY8w+fv3IHl/tf8j2ZJ8uJTJGqzp1RdyLRuMO/XhE6ceX9rz4Mkw/QiBluw NpXYVQ3QaRDlSVL5VetvXYBKBFDiQXd6Rs5sK5nAwA6XCPiiDqW7NqhEzM1Mwz1fDqDc 28P9uCI0dhKWuIPEd5c7mg60SfDIBwITGyYaGgK5M2rc7bwUAyXhKiT3/40bVQU+akXr o7wQ/m8rcYrF33hbShc3uf1vS4HHGjCR5fbjHdQASoCiQ8WYolVR0Rp7Y3oi3P9N655/ NYQuujRMPO9xH/M6IQr9LkWE4QB/2ztImMkiXb1ufwIfZ2RDfxWr+nl70Bi1WOr0cJB6 ig== Received: from userp3020.oracle.com (userp3020.oracle.com [156.151.31.79]) by userp2130.oracle.com with ESMTP id 2rpkhsr0jp-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 08 Apr 2019 17:31:42 +0000 Received: from pps.filterd (userp3020.oracle.com [127.0.0.1]) by userp3020.oracle.com (8.16.0.27/8.16.0.27) with SMTP id x38HTose094106; Mon, 8 Apr 2019 17:31:42 GMT Received: from aserv0122.oracle.com (aserv0122.oracle.com [141.146.126.236]) by userp3020.oracle.com with ESMTP id 2rpkehu6j0-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 08 Apr 2019 17:31:41 +0000 Received: from abhmp0006.oracle.com (abhmp0006.oracle.com [141.146.116.12]) by aserv0122.oracle.com (8.14.4/8.14.4) with ESMTP id x38HVdCW030255; Mon, 8 Apr 2019 17:31:39 GMT Received: from [10.175.213.227] (/10.175.213.227) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Mon, 08 Apr 2019 10:31:39 -0700 Subject: Re: [PATCH RFC 00/39] x86/KVM: Xen HVM guest support To: Juergen Gross Cc: Paolo Bonzini , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Ankur Arora , Boris Ostrovsky , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , x86@kernel.org, Stefano Stabellini , xen-devel@lists.xenproject.org References: <20190220201609.28290-1-joao.m.martins@oracle.com> <35051310-c497-8ad5-4434-1b8426a317d2@redhat.com> <8b1f4912-4f92-69ae-ae01-d899d5640572@oracle.com> <3ee91f33-2973-c2db-386f-afbf138081b4@redhat.com> <59676804-786d-3df8-7752-8e45dec6d65b@oracle.com> <94738323-ebdf-d58e-55b6-313e27c923b0@oracle.com> <585163c2-8dea-728d-7556-9cb3559f0eca@suse.com> From: Joao Martins Message-ID: <97808492-58ee-337f-c894-900b34b7b1a5@oracle.com> Date: Mon, 8 Apr 2019 18:31:35 +0100 MIME-Version: 1.0 In-Reply-To: <585163c2-8dea-728d-7556-9cb3559f0eca@suse.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=9221 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=1 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1904080140 X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=9221 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=1 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 lowpriorityscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1904080140 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 4/8/19 11:42 AM, Juergen Gross wrote: > On 08/04/2019 12:36, Joao Martins wrote: >> On 4/8/19 7:44 AM, Juergen Gross wrote: >>> On 12/03/2019 18:14, Joao Martins wrote: >>>> On 2/22/19 4:59 PM, Paolo Bonzini wrote: >>>>> On 21/02/19 12:45, Joao Martins wrote: >>>>>> On 2/20/19 9:09 PM, Paolo Bonzini wrote: >>>>>>> On 20/02/19 21:15, Joao Martins wrote: >>>>>>>> 2. PV Driver support (patches 17 - 39) >>>>>>>> >>>>>>>> We start by redirecting hypercalls from the backend to routines >>>>>>>> which emulate the behaviour that PV backends expect i.e. grant >>>>>>>> table and interdomain events. Next, we add support for late >>>>>>>> initialization of xenbus, followed by implementing >>>>>>>> frontend/backend communication mechanisms (i.e. grant tables and >>>>>>>> interdomain event channels). Finally, introduce xen-shim.ko, >>>>>>>> which will setup a limited Xen environment. This uses the added >>>>>>>> functionality of Xen specific shared memory (grant tables) and >>>>>>>> notifications (event channels). >>>>>>> >>>>>>> I am a bit worried by the last patches, they seem really brittle and >>>>>>> prone to breakage. I don't know Xen well enough to understand if the >>>>>>> lack of support for GNTMAP_host_map is fixable, but if not, you have to >>>>>>> define a completely different hypercall. >>>>>>> >>>>>> I guess Ankur already answered this; so just to stack this on top of his comment. >>>>>> >>>>>> The xen_shim_domain() is only meant to handle the case where the backend >>>>>> has/can-have full access to guest memory [i.e. netback and blkback would work >>>>>> with similar assumptions as vhost?]. For the normal case, where a backend *in a >>>>>> guest* maps and unmaps other guest memory, this is not applicable and these >>>>>> changes don't affect that case. >>>>>> >>>>>> IOW, the PV backend here sits on the hypervisor, and the hypercalls aren't >>>>>> actual hypercalls but rather invoking shim_hypercall(). The call chain would go >>>>>> more or less like: >>>>>> >>>>>> >>>>>> gnttab_map_refs(map_ops, pages) >>>>>> HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref,...) >>>>>> shim_hypercall() >>>>>> shim_hcall_gntmap() >>>>>> >>>>>> Our reasoning was that given we are already in KVM, why mapping a page if the >>>>>> user (i.e. the kernel PV backend) is himself? The lack of GNTMAP_host_map is how >>>>>> the shim determines its user doesn't want to map the page. Also, there's another >>>>>> issue where PV backends always need a struct page to reference the device >>>>>> inflight data as Ankur pointed out. >>>>> >>>>> Ultimately it's up to the Xen people. It does make their API uglier, >>>>> especially the in/out change for the parameter. If you can at least >>>>> avoid that, it would alleviate my concerns quite a bit. >>>> >>>> In my view, we have two options overall: >>>> >>>> 1) Make it explicit, the changes the PV drivers we have to make in >>>> order to support xen_shim_domain(). This could mean e.g. a) add a callback >>>> argument to gnttab_map_refs() that is invoked for every page that gets looked up >>>> successfully, and inside this callback the PV driver may update it's tracking >>>> page. Here we no longer have this in/out parameter in gnttab_map_refs, and all >>>> shim_domain specific bits would be a little more abstracted from Xen PV >>>> backends. See netback example below the scissors mark. Or b) have sort of a >>>> translate_gref() and put_gref() API that Xen PV drivers use which make it even >>>> more explicit that there's no grant ops involved. The latter is more invasive. >>>> >>>> 2) The second option is to support guest grant mapping/unmapping [*] to allow >>>> hosting PV backends inside the guest. This would remove the Xen changes in this >>>> series completely. But it would require another guest being used >>>> as netback/blkback/xenstored, and less performance than 1) (though, in theory, >>>> it would be equivalent to what does Xen with grants/events). The only changes in >>>> Linux Xen code is adding xenstored domain support, but that is useful on its own >>>> outside the scope of this work. >>>> >>>> I think there's value on both; 1) is probably more familiar for KVM users >>>> perhaps (as it is similar to what vhost does?) while 2) equates to implementing >>>> Xen disagregation capabilities in KVM. >>>> >>>> Thoughts? Xen maintainers what's your take on this? >>> >>> What I'd like best would be a new handle (e.g. xenhost_t *) used as an >>> abstraction layer for this kind of stuff. It should be passed to the >>> backends and those would pass it on to low-level Xen drivers (xenbus, >>> event channels, grant table, ...). >>> >> So if IIRC backends would use the xenhost layer to access grants or frames >> referenced by grants, and that would grok into some of this. IOW, you would have >> two implementors of xenhost: one for nested remote/local events+grants and >> another for this "shim domain" ? > > As I'd need that for nested Xen I guess that would make it 3 variants. > Probably the xen-shim variant would need more hooks, but that should be > no problem. > I probably messed up in the short description but "nested remote/local events+grants" was referring to nested Xen (FWIW remote meant L0 and local L1). So maybe only 2 variants are needed? >>> I was planning to do that (the xenhost_t * stuff) soon in order to add >>> support for nested Xen using PV devices (you need two Xenstores for that >>> as the nested dom0 is acting as Xen backend server, while using PV >>> frontends for accessing the "real" world outside). >>> >>> The xenhost_t should be used for: >>> >>> - accessing Xenstore >>> - issuing and receiving events >>> - doing hypercalls >>> - grant table operations >>> >> >> In the text above, I sort of suggested a slice of this on 1.b) with a >> translate_gref() and put_gref() API -- to get the page from a gref. This was >> because of the flags|host_addr hurdle we depicted above wrt to using using grant >> maps/unmaps. You think some of the xenhost layer would be ammenable to support >> this case? > > I think so, yes. > >> >>> So exactly the kind of stuff you want to do, too. >>> >> Cool idea! > > In the end you might make my life easier for nested Xen. :-) > Hehe :) > Do you want to have a try with that idea or should I do that? I might be > able to start working on that in about a month. > Ankur (CC'ed) will give a shot at it, and should start a new thread on this xenhost abstraction layer. Joao