On Tue, 25 Aug 2020 09:15:04 +0100 "Dr. David Alan Gilbert" <dgilb...@redhat.com> wrote:
> * Babu Moger (babu.mo...@amd.com) wrote: > > Hi Dave, > > > > On 8/24/20 1:41 PM, Dr. David Alan Gilbert wrote: > > > * Babu Moger (babu.mo...@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 >
