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Fri, 28 Aug 2020 16:32:54 +0000 (UTC) Date: Fri, 28 Aug 2020 17:32:51 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Eduardo Habkost Subject: Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode Message-ID: <20200828163251.GQ224144@redhat.com> References: <20200826125059.GN168515@redhat.com> <20200826153034.115126cb@redhat.com> <20200826133638.GO168515@redhat.com> <20200826160258.0e9047f4@redhat.com> <20200826150340.GP168515@redhat.com> <20200827190314.717ec788@imammedo-mac> <20200827190752.GK642093@habkost.net> <20200827225526.0b1f6d32@imammedo-mac> <20200828085533.GC224144@redhat.com> <20200828162931.GL642093@habkost.net> MIME-Version: 1.0 In-Reply-To: <20200828162931.GL642093@habkost.net> User-Agent: Mutt/1.14.6 (2020-07-11) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=berrange@redhat.com X-Mimecast-Spam-Score: 0.002 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Disposition: inline Received-SPF: pass client-ip=207.211.31.81; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/08/28 12:33:08 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_H3=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: mst@redhat.com, Michal Privoznik , qemu-devel@nongnu.org, Babu Moger , pbonzini@redhat.com, Igor Mammedov , rth@twiddle.net Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, Aug 28, 2020 at 12:29:31PM -0400, Eduardo Habkost wrote: > On Fri, Aug 28, 2020 at 09:55:33AM +0100, Daniel P. Berrangé wrote: > > On Thu, Aug 27, 2020 at 10:55:26PM +0200, Igor Mammedov wrote: > > > On Thu, 27 Aug 2020 15:07:52 -0400 > > > Eduardo Habkost wrote: > > > > > > > On Thu, Aug 27, 2020 at 07:03:14PM +0200, Igor Mammedov wrote: > > > > > On Wed, 26 Aug 2020 16:03:40 +0100 > > > > > Daniel P. Berrangé wrote: > > > > > > > > > > > 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. > > > > > I find (3) and (4) good enough reasons to use deprecation. > > > > > > > > > > > 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. > > > > > > > > > > (1) for configs that started without -numa &&|| without -smp dies>1, > > > > > QEMU can do just that (enable auto_enable_numa). > > > > > > > > Why exactly do we need auto_enable_numa with dies=1? > > > > > > > > If I understand correctly, Babu said earlier in this thread[1] > > > > that we don't need auto_enable_numa. > > > > > > > > [1] https://lore.kernel.org/qemu-devel/11489e5f-2285-ddb4-9c35-c9f522d603a0@amd.com/ > > > > > > in case of 1 die, -numa is not must have as it's one numa node only. > > > Though having auto_enable_numa, will allow to reuse the CPU.node-id property > > > to compose CPUID_Fn8000001E_ECX. i.e only code one path vs numa|non-numa variant. > > > > > > > > > > > (2) As for configs that are out of spec, I do not care much (junk in - junk out) > > > > > (though not having to spend time on bug reports and debug issues, just to say > > > > > it's not supported in the end, makes deprecation sound like a reasonable > > > > > choice) > > > > > > > > > > (3) However if config matches bare metal i.e. CPU has more than 1 die and within > > > > > dies limits (spec wise), QEMU has to produce valid CPUs. > > > > > In this case QEMU can't make up multiple numa nodes and mappings of RAM/CPUs > > > > > on user's behalf. That's where we have to error out and ask for explicit > > > > > numa configuration. > > > > > > > > > > For such configs, current code (since 5.0), will produce in the best case > > > > > performance issues due to mismatching data in APICID, CPUID and ACPI tables, > > > > > in the worst case issues might be related to invalid APIC ID if running on EPYC host > > > > > and HW takes in account subfields of APIC ID (according to Babu real CPU uses > > > > > die_id(aka node_id) internally). > > > > > I'd rather error out on nonsense configs earlier than debug such issues > > > > > and than error out anyways later (upsetting more users). > > > > > > > > > > > > > The requirements are not clear to me. Is this just about making > > > > CPU die_id match the NUMA node ID, or are there additional > > > > constraints? > > > die_id is per socket numa node index, so it's not numa node id in > > > a sense we use it in qemu > > > (that's where all the confusion started that led to current code) > > > > > > I understood that each die in EPYC chip is a numa node, which encodes > > > NUMA node ID (system wide) in CPUID_Fn8000001E_ECX, that's why I > > > wrote earlier that EPYC makes -numa non optional. > > > > AFAIK, that isnt a hard requirement. In bare metal EPYC machine I > > have used, the BIOS lets you choose whether the dies are exposed as > > 1, 2 or 4 NUMA nodes. So there's no fixed die == numa node mapping > > that I see. > > If you change that setting, will all CPUID bits be kept the same, > or the die topology seen by the OS will change? I don't know offhand, and don't currently have access to the hardware. All I know is that I was able to change between 1, 2 and 4 NUMA nodes and that was reflected in numactl display, I didn't check the CPUID when I was testing previously. 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 :|