Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode

[Dr. David Alan Gilbert]

* Igor Mammedov (imammedo@redhat.com) wrote:
> On Tue, 25 Aug 2020 16:25:21 +0100
> "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> 
> > * Igor Mammedov (imammedo@redhat.com) wrote:
> > > On Tue, 25 Aug 2020 09:15:04 +0100
> > > "Dr. David Alan Gilbert" <dgilbert@redhat.com> 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&amp;data=02%7C01%7Cbabu.moger%40amd.com%7C74d90724af9c4adcc75008d8485d4d16%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637338912853492167&amp;sdata=GTsMKcpeYXAA0CvpLTirPHKdNSdlJE3RuPjCtSyWtGQ%3D&amp;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?

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
> > > >   
> > >   
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK