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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 6390BC433FF for ; Wed, 31 Jul 2019 13:56:32 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (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 3A6CA20693 for ; Wed, 31 Jul 2019 13:56:32 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3A6CA20693 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:41054 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.86_2) (envelope-from ) id 1hsp5j-00068q-4s for qemu-devel@archiver.kernel.org; Wed, 31 Jul 2019 09:56:31 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:57614) by lists.gnu.org with esmtp (Exim 4.86_2) (envelope-from ) id 1hsp5J-0005kH-Sq for qemu-devel@nongnu.org; Wed, 31 Jul 2019 09:56:07 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hsp5I-0005Qm-IO for qemu-devel@nongnu.org; Wed, 31 Jul 2019 09:56:05 -0400 Received: from mx1.redhat.com ([209.132.183.28]:50872) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1hsp5I-0005Q5-Ae for qemu-devel@nongnu.org; Wed, 31 Jul 2019 09:56:04 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 441978B134; Wed, 31 Jul 2019 13:56:03 +0000 (UTC) Received: from gondolin (dhcp-192-232.str.redhat.com [10.33.192.232]) by smtp.corp.redhat.com (Postfix) with ESMTP id 757655D9CA; Wed, 31 Jul 2019 13:55:53 +0000 (UTC) Date: Wed, 31 Jul 2019 15:55:51 +0200 From: Cornelia Huck To: "Michael S. Tsirkin" Message-ID: <20190731155551.4bb57ec3.cohuck@redhat.com> In-Reply-To: <20190730160605-mutt-send-email-mst@kernel.org> References: <20190729125755.45008-1-slp@redhat.com> <20190730133546.056f8b19.cohuck@redhat.com> <09e5ceb5e7c03f74f05307a3b9f9a4df035ff74f.camel@redhat.com> <20190730151400.20686a5b.cohuck@redhat.com> <20190730160605-mutt-send-email-mst@kernel.org> Organization: Red Hat GmbH MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.28]); Wed, 31 Jul 2019 13:56:03 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] [RFC] virtio-mmio: implement modern (v2) personality (virtio-1) X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: peter.maydell@linaro.org, Andrea Bolognani , Sergio Lopez , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Tue, 30 Jul 2019 16:18:52 -0400 "Michael S. Tsirkin" wrote: > On Tue, Jul 30, 2019 at 03:14:00PM +0200, Cornelia Huck wrote: > > On Tue, 30 Jul 2019 14:17:48 +0200 > > Andrea Bolognani wrote: > > > > > On Tue, 2019-07-30 at 13:35 +0200, Cornelia Huck wrote: > > > > On Tue, 30 Jul 2019 12:25:30 +0200 > > > > Andrea Bolognani wrote: > > > > > Can you please make sure virtio-mmio uses the existing interface > > > > > instead of introducing a new one? > > > > > > > > FWIW, I really hate virtio-pci's disable-modern/disable-legacy... for a > > > > starter, what is 'modern'? Will we have 'ultra-modern' in the future? > > > > > > AIUI the modern/legacy terminology is part of the VirtIO spec, so > > > while I agree that it's not necessarily the least prone to ambiguity > > > at least it's well defined. > > > > Legacy is, modern isn't :) Devices/drivers are conforming to the > > standard, I don't think there's a special term for that. > > Right, if we followed the spec, disable-modern would have been > force-legacy. > > I'm fine with adding force-legacy for everyone and asking tools to > transition if there. Document it's same as disable-modern for pci. > Cornelia? 'force-legacy' is certainly better than 'disable-modern'. Not sure if it's much of a gain at this point in time, and it does not really add anything over limiting the revision to 0 for ccw, but I don't really object to it. > > > > > > > > > It is also quite backwards with the 'disable' terminology. > > > > > > That's also true. I never claimed the way virtio-pci does it is > > > perfect! > > > > > > > We also have a different mechanism for virtio-ccw ('max_revision', > > > > which covers a bit more than virtio-1; it doesn't have a 'min_revision', > > > > as negotiating the revision down is fine), so I don't see why > > > > virtio-mmio should replicate the virtio-pci mechanism. > > > > > > > > Also, IIUC, virtio-mmio does not have transitional devices, but either > > > > version 1 (legacy) or version 2 (virtio-1). It probably makes more > > > > sense to expose the device version instead; either as an exact version > > > > (especially if it isn't supposed to go up without incompatible > > > > changes), or with some min/max concept (where version 1 would stand a > > > > bit alone, so that would probably be a bit awkward.) > > > > > > I think that if reinventing the wheel is generally agreed not to be > > > a good idea, then it stands to reason that reinventing it twice can > > > only be described as absolute madness :) > > > > > > We should have a single way to control the VirtIO protocol version > > > that works for all VirtIO devices, regardless of transport. We might > > > even want to have virtio-*-{device,ccw}-non-transitional to mirror > > > the existing virtio-*-pci-non-transitional. > > > > > > FWIW, libvirt already implements support for (non)-transitional > > > virtio-pci devices using either the dedicated devices or the base > > > virtio-pci plus the disable-{modern,legacy} attributes. > > > > One problem (besides my dislike of the existing virtio-pci > > interfaces :) is that pci, ccw, and mmio all have slightly different > > semantics. > > > > - pci: If we need to keep legacy support around, we cannot enable some > > features (IIRC, pci-e, maybe others as well.) That means transitional > > devices are in some ways inferior to virtio-1 only devices, so it > > makes a lot of sense to be able to configure devices without legacy > > support. The differences between legacy and virtio-1 are quite large. > > - ccw: Has revisions negotiated between device and driver; virtio-1 > > requires revision 1 or higher. (Legacy drivers that don't know the > > concept of revisions automatically get revision 0.) Differences > > between legacy and virtio-1 are mostly virtqueue endianness and some > > control structures. > > - mmio: Has device versions offered by the device, the driver can take > > it or leave it. No transitional devices. Differences don't look as > > large as the ones for pci, either. > > > > So, if we were to duplicate the same scheme as for pci for ccw and mmio > > as well, we'd get > > > > - ccw: devices that support revision 0 only (disable-modern), that act > > as today, or that support at least revision 1 (disable-legacy). We > > still need to keep max_revision around for backwards compatibility. > > Legacy only makes sense for compat machines (although this is > > equivalent to max_revision 0); I don't see a reason why you would > > want virtio-1 only devices, unless you'd want to rip out legacy > > support in QEMU completely. > > Reduce security attack surface slightly. Save some cycles > (down the road) on branches in the endian-ness handling. Most of that stuff is actually in the core code, right? Ripping out legacy will have much more impact outside of ccw, I guess. > Make sure your guests > are all up to date in preparation to the day when legacy will go away. If legacy goes away, legacy guests will be busted anyway :) (There should not be many, if any, of these -- ccw switched on virtio-1 by default quite some time ago, and the s390x legacy virtio transport was s390-virtio anyway :) > > Not a huge win, for sure, but hey - it's something. > > > - mmio: devices that support version 1 (disable-modern), or version 2 > > (disable-legacy). You cannot have both at the same time. Whether this > > makes sense depends on whether there will be a version 3 in the > > future. > > > > So, this might make some sense for mmio; for ccw, I don't see any > > advantages other than confusing people further...