On Fri, 28 Aug 2020 09:48:30 +0100 "Dr. David Alan Gilbert" <dgilb...@redhat.com> wrote:
> * Igor Mammedov (imamm...@redhat.com) wrote: > > On Wed, 26 Aug 2020 15:10:46 +0100 > > "Dr. David Alan Gilbert" <dgilb...@redhat.com> wrote: > > > > > * Igor Mammedov (imamm...@redhat.com) wrote: > > > > 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? > > > > > > We have no way to block based on version - and that's a pretty painful > > > thing to do; we can block based on machine type. > > > > > > But before we get there; can we understand in which combinations that > > > things break and why exactly - would it break on a 1 or 2 vCPU guest - > > > or would it only break when we get to the point the upper bits start > > > being used for example? Why exaclty would it break - i.e. is it going > > > to change the name of sections in the migration stream - or are the > > > values we need actually going to migrate OK? > > > > it's values of APIC ID, where 4.2 and 5.0 QEMU use different values > > if numa is enabled. > > I'd expect guest to be very confused in when this happens. > > > > here is an example: > > qemu-4.2 -cpu EPYC -smp 8,sockets=1,cores=8 -numa node,cpus=0-3 -numa > > node,cpus=4-7 > > OK, but it'll probably be OK on small VMs with a single NUMA node? it should be fine if -numa isn't used. > Dave > > > (QEMU) qom-get path=/machine/unattached/device[8] property=apic-id > > { > > "return": 7 > > } > > > > vs > > > > qemu-5.1 -cpu EPYC -smp 8,sockets=1,cores=8 -numa node,cpus=0-3 -numa > > node,cpus=4-7 > > (QEMU) qom-get path=/machine/unattached/device[8] property=apic-id > > { > > "return": 15 > > } > > > > we probably can't do anything based on machine type versions, as > > 4.2 and older versions on qemu-5.0 and newer use different algorithm to > > calculate apic-id. > > > > Hence was suggestion to leave 5.0/5.1 with broken apic id and revert back to > > 4.2 algorithm, which should encode APIC ID correctly when '-smp dies' is > > used. > > > > > > > Dave > > > > > > > > > > > > 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 > > > > > > > > > > > > > > > > > > >
