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Tue, 25 Aug 2020 14:38:47 +0000 (UTC) Date: Tue, 25 Aug 2020 16:38:46 +0200 From: Igor Mammedov To: "Dr. David Alan Gilbert" Subject: Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode Message-ID: <20200825163846.10185087@redhat.com> In-Reply-To: <20200825081504.GA2646@work-vm> References: <159804762216.39954.15502128500494116468.stgit@naples-babu.amd.com> <20200824184112.GB2688@work-vm> <20200825081504.GA2646@work-vm> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=imammedo@redhat.com X-Mimecast-Spam-Score: 0.002 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=207.211.31.81; envelope-from=imammedo@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/08/25 01:36:47 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=-0.958, 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_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: ehabkost@redhat.com, mst@redhat.com, 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 Tue, 25 Aug 2020 09:15:04 +0100 "Dr. David Alan Gilbert" wrote: > * Babu Moger (babu.moger@amd.com) wrote: > > Hi Dave, > > > > On 8/24/20 1:41 PM, Dr. David Alan Gilbert wrote: > > > * Babu Moger (babu.moger@amd.com) 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%2Flore.kernel.org%2Fqemu-devel%2Fc0bcc1a6-1d84-a6e7-e468-d5b437c1b254%40amd.com%2F&data=02%7C01%7Cbabu.moger%40amd.com%7C74d90724af9c4adcc75008d8485d4d16%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637338912853492167&sdata=GTsMKcpeYXAA0CvpLTirPHKdNSdlJE3RuPjCtSyWtGQ%3D&reserved=0 > > >> > > >> This series removes all the EPYC mode specific apicid changes and use the generic > > >> apicid decode. > > > > > > Hi Babu, > > > This does simplify things a lot! > > > One worry, what happens about a live migration of a VM from an old qemu > > > that was using the node-id to a qemu with this new scheme? > > > > The node_id which we introduced was only used internally. This wasn't > > exposed outside. I don't think live migration will be an issue. > > Didn't it become part of the APIC ID visible to the guest? Daniel asked similar question wrt hard error on start up, when CLI is not sufficient to create EPYC cpu. https://www.mail-archive.com/qemu-devel@nongnu.org/msg728536.html Migration might fall into the same category. Also looking at the history, 5.0 commit 247b18c593ec29 target/i386: Enable new apic id encoding for EPYC based cpus models silently broke APIC ID (without versioning), for all EPYC models (that's were 1 new and 1 old one). (I'm not aware of somebody complaining about it) Another commit ed78467a21459, changed CPUID_8000_001E without versioning as well. With current EPYC apicid code, if all starts align (no numa or 1 numa node only on CLI and no -smp dies=) it might produce a valid CPU (apicid+CPUID_8000_001E). No numa is gray area, since EPYC spec implies that it has to be numa machine in case of real EPYC cpus. Multi-node configs would be correct only if user assigns cpus to numa nodes by duplicating internal node_id algorithm that this series removes. There might be other broken cases that I don't recall anymore (should be mentioned in previous versions of this series) To summarize from migration pov (ignoring ed78467a21459 change): 1) old qemu pre-5.0 ==> qemu 5.0, 5.1 - broken migration 2) with this series (lets call it qemu 5.2) pre-5.0 ==> qemu 5.2 - should work as series basically rollbacks current code to pre-5.0 qemu 5.0, 5.1 ==> qemu 5.2 - broken It's all about picking which poison to choose, I'd preffer 2nd case as it lets drop a lot of complicated code that doesn't work as expected. PS: I didn't review it yet, but with this series we aren't making up internal node_ids that should match user provided numa node ids somehow. It seems series lost the patch that would enforce numa in case -smp dies>1, but otherwise it heads in the right direction. > > Dave >