On Tue, 25 Aug 2020 16:25:21 +0100 "Dr. David Alan Gilbert" <dgilb...@redhat.com> wrote:
> * Igor Mammedov (imamm...@redhat.com) wrote: > > 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 > > Oh .... > > > 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. > > I think that would make our lives easier for other reasons; so I'm happy > to go with that. to make things less painful for users, me wonders if there is a way to block migration if epyc and specific QEMU versions are used? > > 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 > > > > > > > Dave > > > > >
