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=-5.1 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,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 EB84FC433ED for ; Wed, 21 Apr 2021 09:37:53 +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 2034161448 for ; Wed, 21 Apr 2021 09:37:53 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2034161448 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]:57486 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lZ9Is-0003GL-Rq for qemu-devel@archiver.kernel.org; Wed, 21 Apr 2021 05:37:50 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:41100) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lZ9FU-0000SG-DE for qemu-devel@nongnu.org; Wed, 21 Apr 2021 05:34:20 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:56407) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lZ9FR-0005f2-PY for qemu-devel@nongnu.org; Wed, 21 Apr 2021 05:34:19 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1618997657; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=RFpR4VOrHJGBsu4tx+kk7NE3iXDhiUpGsrJYVXmMIMI=; b=WtpxtnWDXLlDqRkDX2E1lISZ+5sba8PGNTJ55z8IAMKv6wQg8ZbVPrzHSXrc7cz7/yfXkR vvs/kw5npiF2IGVqJN4uO1PPk9Bq0l8+c4tZkEJqlkDvPjmr5Q7NgJfGkjPzGAOb09Jkba kW1/dCtfKxqbNKNl3376bu9LkmC3/Oo= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-334-lzwtolHPMHCIrFmU5vwIWA-1; Wed, 21 Apr 2021 05:34:15 -0400 X-MC-Unique: lzwtolHPMHCIrFmU5vwIWA-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 7362483DD2A; Wed, 21 Apr 2021 09:34:14 +0000 (UTC) Received: from work-vm (ovpn-114-29.ams2.redhat.com [10.36.114.29]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 5DA416061F; Wed, 21 Apr 2021 09:34:09 +0000 (UTC) Date: Wed, 21 Apr 2021 10:34:06 +0100 From: "Dr. David Alan Gilbert" To: Vitaly Kuznetsov Subject: Re: [PATCH 0/2] i386: Fix interrupt based Async PF enablement Message-ID: References: <20210401151957.408028-1-vkuznets@redhat.com> <87sg43d4gk.fsf@vitty.brq.redhat.com> <091dec53-cc0c-04ca-154a-3cfab6475705@redhat.com> <20210420173541.24kxarhw75h2rqxa@habkost.net> <8735vkhw29.fsf@vitty.brq.redhat.com> <87zgxsgf3q.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 In-Reply-To: <87zgxsgf3q.fsf@vitty.brq.redhat.com> User-Agent: Mutt/2.0.6 (2021-03-06) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dgilbert@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=dgilbert@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action 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: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , Eduardo Habkost , "Michael S. Tsirkin" , Richard Henderson , qemu-devel@nongnu.org, Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Vitaly Kuznetsov (vkuznets@redhat.com) wrote: > Daniel P. Berrangé writes: > > > On Wed, Apr 21, 2021 at 10:38:06AM +0200, Vitaly Kuznetsov wrote: > >> Eduardo Habkost writes: > >> > >> > On Thu, Apr 15, 2021 at 08:14:30PM +0100, Dr. David Alan Gilbert wrote: > >> >> * Paolo Bonzini (pbonzini@redhat.com) wrote: > >> >> > On 06/04/21 13:42, Vitaly Kuznetsov wrote: > >> >> > > older machine types are still available (I disable it for <= 5.1 but we > >> >> > > can consider disabling it for 5.2 too). The feature is upstream since > >> >> > > Linux 5.8, I know that QEMU supports much older kernels but this doesn't > >> >> > > probably mean that we can't enable new KVM PV features unless all > >> >> > > supported kernels have it, we'd have to wait many years otherwise. > >> >> > > >> >> > Yes, this is a known problem in fact. :( In 6.0 we even support RHEL 7, > >> >> > though that will go away in 6.1. > >> >> > > >> >> > We should take the occasion of dropping RHEL7 to be clearer about which > >> >> > kernels are supported. > >> >> > >> >> It would be nice to be able to define sets of KVM functonality that we > >> >> can either start given machine types with, or provide a separate switch > >> >> to limit kvm functionality back to some defined point. We do trip over > >> >> the same things pretty regularly when accidentally turning on new > >> >> features. > >> > > >> > The same idea can apply to the hyperv=on stuff Vitaly is working > >> > on. Maybe we should consider making a generic version of the > >> > s390x FeatGroup code, use it to define convenient sets of KVM and > >> > hyperv features. > >> > >> True, the more I look at PV features enablement, the more I think that > >> we're missing something important in the logic. All machine types we > >> have are generally suposed to work with the oldest supported kernel so > >> we should wait many years before enabling some of the new PV features > >> (KVM or Hyper-V) by default. > >> > >> This also links to our parallel discussion regarding migration > >> policies. Currently, we can't enable PV features by default based on > >> their availability on the host because of migration, the set may differ > >> on the destination host. What if we introduce (and maybe even switch to > >> it by default) something like > >> > >> -migratable opportunistic (stupid name, I know) > >> > >> which would allow to enable all features supported by the source host > >> and then somehow checking that the destination host has them all. This > >> would effectively mean that it is possible to migrate a VM to a > >> same-or-newer software (both kernel an QEMU) but not the other way > >> around. This may be a reasonable choice. > > > > I don't think this is usable in pratice. Any large cloud or data center > > mgmt app using QEMU relies on migration, so can't opportunistically > > use arbitrary new features. They can only use features in the oldest > > kernel their deployment cares about. This can be newer than the oldest > > that QEMU supports, but still older than the newest that exists. > > > > ie we have situation where: > > > > - QEMU upstream minimum host is version 7 > > - Latest possible host is version 45 > > - A particular deployment has a mixture of hosts at version 24 and 37 > > > > "-migratable opportunistic" would let QEMU use features from version 37 > > despite the deployment needing compatibility with host version 24 still. > > > > True; I was not really thinking about 'big' clouds/data centers, these > should have enough resources to carefully set all the required features > and not rely on the 'default'. My thoughts were around using migration > for host upgrade on smaller (several hosts) deployments and in this case > it's probably fairly reasonable to require to start with the oldest host > and upgrade them all if getting new features is one of the upgrade goals. It's not actually that simple. Small installations tend to have less spare hardware available and/or flexibility; if you've got say a 3 or 5 host cluster, once you start upgrading one node you've now got nowhere to go if you hit a problem. Dave > > > > It is almost as if we need to have a way to explicitly express a minimum > > required host version that VM requires compatibility with, so deployments > > can set their own baseline that is newer than QEMU minimum. > > Yes, maybe, but setting the baseline is also a non-trivial task: > e.g. how would users know which PV features they can enable without > going through Linux kernel logs or just trying them on the oldest kernel > they need? This should probably be solved by some upper layer management > app which would collect feature sets from all hosts and come up with a > common subset. I'm not sure if this is done by some tools already. > > -- > Vitaly > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK