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Wed, 26 Aug 2020 17:18:01 +0000 From: Babu Moger Subject: RE: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode To: Igor Mammedov , =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= References: <159804762216.39954.15502128500494116468.stgit@naples-babu.amd.com> <20200826143849.59f6970b@redhat.com> <20200826125059.GN168515@redhat.com> <20200826153034.115126cb@redhat.com> Message-ID: <5c00dac9-5ea5-405c-93be-1ac903ddff62@amd.com> Date: Wed, 26 Aug 2020 12:17:59 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 In-Reply-To: <20200826153034.115126cb@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-ClientProxiedBy: SA0PR11CA0038.namprd11.prod.outlook.com (2603:10b6:806:d0::13) To SN1PR12MB2560.namprd12.prod.outlook.com (2603:10b6:802:26::19) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from 255.255.255.255 (255.255.255.255) by SA0PR11CA0038.namprd11.prod.outlook.com (2603:10b6:806:d0::13) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3326.19 via Frontend Transport; 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envelope-from=Babu.Moger@amd.com; helo=NAM11-BN8-obe.outbound.protection.outlook.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/08/26 13:33:05 X-ACL-Warn: Detected OS = Windows NT kernel [generic] [fuzzy] X-Spam_score_int: -17 X-Spam_score: -1.8 X-Spam_bar: - X-Spam_report: (-1.8 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, FORGED_SPF_HELO=1, MSGID_FROM_MTA_HEADER=0.001, NICE_REPLY_A=-2.239, RCVD_ILLEGAL_IP=1.3, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_PASS=-0.001, SPF_NONE=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: "ehabkost@redhat.com" , "mst@redhat.com" , Michal Privoznik , "qemu-devel@nongnu.org" , "pbonzini@redhat.com" , "rth@twiddle.net" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" > -----Original Message----- > From: Igor Mammedov > Sent: Wednesday, August 26, 2020 8:31 AM > To: Daniel P. Berrangé > Cc: Moger, Babu ; pbonzini@redhat.com; > rth@twiddle.net; ehabkost@redhat.com; qemu-devel@nongnu.org; > mst@redhat.com; Michal Privoznik > Subject: Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic > decode > > 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://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2F > > > > lore.kernel.org%2Fqemu-devel%2Fc0bcc1a6-1d84-a6e7-e468- > d5b437c1b25 > > > > > 4%40amd.com%2F&data=02%7C01%7Cbabu.moger%40amd.com%7C8a5c > 52f92 > > > > > 3f04082a40808d849c43d49%7C3dd8961fe4884e608e11a82d994e183d%7C0%7 > C0 > > > > > %7C637340454473508873&sdata=VnW28H1v4XwK3GaNGFxu%2BhwiMeA > YO%2B > > > > 3WAzo3DeY5Ha8%3D&reserved=0 > > > > > > > > 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 No, That is not true. Because of that assumption we made all these apicid changes. And here we are now. AMD supports varies mixed configurations. In case of EPYC-Rome, we have NPS1, NPS2 and NPS4(Numa Nodes per socket). In case of NPS1, basically we have all the cores in a socket under one numa node. This is non-numa configuration. Looking at the various configurations and also discussing internally, it is not advisable to have (epyc && !numa) check. > > > 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. > > > > > > Regards, > > Daniel