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bh=Xcup8dxjZpWexDTgtKAr1RgveIuoirchEhVIQgNrxGA=; b=Gc7A9hxVA03ZvYPR9K68ov8EcHh92iFfnEMDBI27RTPfyMJ5o6+ngI9QPGw9XNdMRNTsO6 5oLkg402B0M7QlT3J0iBJYPZoJtI4hODtx1O/9TpqDgHkjrAixWoTCQ2lrMJulV38d4ab7 mTh/5jGldzNh30edsUuk+COtnrfDouo= 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-386-rnHCWsS-PiiwcNwJy-hkkg-1; Wed, 26 Aug 2020 11:03:49 -0400 X-MC-Unique: rnHCWsS-PiiwcNwJy-hkkg-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 780BF10082E5; Wed, 26 Aug 2020 15:03:48 +0000 (UTC) Received: from redhat.com (ovpn-114-142.ams2.redhat.com [10.36.114.142]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 87AB26198B; Wed, 26 Aug 2020 15:03:43 +0000 (UTC) Date: Wed, 26 Aug 2020 16:03:40 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Igor Mammedov Subject: Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode Message-ID: <20200826150340.GP168515@redhat.com> References: <159804762216.39954.15502128500494116468.stgit@naples-babu.amd.com> <20200826143849.59f6970b@redhat.com> <20200826125059.GN168515@redhat.com> <20200826153034.115126cb@redhat.com> <20200826133638.GO168515@redhat.com> <20200826160258.0e9047f4@redhat.com> MIME-Version: 1.0 In-Reply-To: <20200826160258.0e9047f4@redhat.com> User-Agent: Mutt/1.14.6 (2020-07-11) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=berrange@redhat.com X-Mimecast-Spam-Score: 0.001 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Disposition: inline Received-SPF: pass client-ip=63.128.21.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/08/26 06:53:10 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.959, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: ehabkost@redhat.com, mst@redhat.com, Michal Privoznik , qemu-devel@nongnu.org, Babu Moger , pbonzini@redhat.com, rth@twiddle.net Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Aug 26, 2020 at 04:02:58PM +0200, Igor Mammedov wrote: > On Wed, 26 Aug 2020 14:36:38 +0100 > Daniel P. Berrangé wrote: > > > On Wed, Aug 26, 2020 at 03:30:34PM +0200, Igor Mammedov wrote: > > > On Wed, 26 Aug 2020 13:50:59 +0100 > > > Daniel P. Berrangé wrote: > > > > > > > On Wed, Aug 26, 2020 at 02:38:49PM +0200, Igor Mammedov wrote: > > > > > On Fri, 21 Aug 2020 17:12:19 -0500 > > > > > Babu Moger wrote: > > > > > > > > > > > To support some of the complex topology, we introduced EPYC mode apicid decode. > > > > > > But, EPYC mode decode is running into problems. Also it can become quite a > > > > > > maintenance problem in the future. So, it was decided to remove that code and > > > > > > use the generic decode which works for majority of the topology. Most of the > > > > > > SPECed configuration would work just fine. With some non-SPECed user inputs, > > > > > > it will create some sub-optimal configuration. > > > > > > Here is the discussion thread. > > > > > > https://lore.kernel.org/qemu-devel/c0bcc1a6-1d84-a6e7-e468-d5b437c1b254@amd.com/ > > > > > > > > > > > > This series removes all the EPYC mode specific apicid changes and use the generic > > > > > > apicid decode. > > > > > > > > > > the main difference between EPYC and all other CPUs is that > > > > > it requires numa configuration (it's not optional) > > > > > so we need an extra patch on top of this series to enfoce that, i.e: > > > > > > > > > > if (epyc && !numa) > > > > > error("EPYC cpu requires numa to be configured") > > > > > > > > Please no. This will break 90% of current usage of the EPYC CPU in > > > > real world QEMU deployments. That is way too user hostile to introduce > > > > as a requirement. > > > > > > > > Why do we need to force this ? People have been successfuly using > > > > EPYC CPUs without NUMA in QEMU for years now. > > > > > > > > It might not match behaviour of bare metal silicon, but that hasn't > > > > obviously caused the world to come crashing down. > > > So far it produces warning in linux kernel (RHBZ1728166), > > > (resulting performance might be suboptimal), but I haven't seen > > > anyone reporting crashes yet. > > > > > > > > > What other options do we have? > > > Perhaps we can turn on strict check for new machine types only, > > > so old configs can keep broken topology (CPUID), > > > while new ones would require -numa and produce correct topology. > > > > No, tieing this to machine types is not viable either. That is still > > going to break essentially every single management application that > > exists today using QEMU. > for that we have deprecation process, so users could switch to new CLI > that would be required. We could, but I don't find the cost/benefit tradeoff is compelling. There are so many places where we diverge from what bare metal would do, that I don't see a good reason to introduce this breakage, even if we notify users via a deprecation message. If QEMU wants to require NUMA for EPYC, then QEMU could internally create a single NUMA node if none was specified for new machine types, such that there is no visible change or breakage to any mgmt apps. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|