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

[Igor Mammedov]

On Wed, 26 Aug 2020 13:50:59 +0100
Daniel P. Berrangé <berrange@redhat.com> wrote:

> On Wed, Aug 26, 2020 at 02:38:49PM +0200, Igor Mammedov wrote:
> > On Fri, 21 Aug 2020 17:12:19 -0500
> > 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.
> > > c0bcc1a6-1d84-a6e7-e468-d5b437c1b254@amd.com/">https://lore.kernel.org/qemu-devel/c0bcc1a6-1d84-a6e7-e468-d5b437c1b254@amd.com/
> > > 
> > > This series removes all the EPYC mode specific apicid changes and use the 
> > > generic
> > > apicid decode.  
> > 
> > the main difference between EPYC and all other CPUs is that
> > it requires numa configuration (it's not optional)
> > so we need an extra patch on top of this series to enfoce that, i.e:
> > 
> >  if (epyc && !numa) 
> >     error("EPYC cpu requires numa to be configured")  
> 
> Please no. This will break 90% of current usage of the EPYC CPU in
> real world QEMU deployments. That is way too user hostile to introduce
> as a requirement.
> 
> Why do we need to force this ?  People have been successfuly using
> EPYC CPUs without NUMA in QEMU for years now.
> 
> It might not match behaviour of bare metal silicon, but that hasn't
> obviously caused the world to come crashing down.
So far it produces warning in linux kernel (RHBZ1728166),
(resulting performance might be suboptimal), but I haven't seen
anyone reporting crashes yet.


What other options do we have?
Perhaps we can turn on strict check for new machine types only,
so old configs can keep broken topology (CPUID),
while new ones would require -numa and produce correct topology.


> 
> Regards,
> Daniel