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

[Igor Mammedov]

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
 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
>