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

[Babu Moger]

> -----Original Message-----
> From: Dr. David Alan Gilbert <dgilbert@redhat.com>
> Sent: Wednesday, August 26, 2020 1:34 PM
> To: Moger, Babu <Babu.Moger@amd.com>
> Cc: Igor Mammedov <imammedo@redhat.com>; Daniel P. Berrangé
> <berrange@redhat.com>; ehabkost@redhat.com; mst@redhat.com; Michal
> Privoznik <mprivozn@redhat.com>; qemu-devel@nongnu.org;
> pbonzini@redhat.com; rth@twiddle.net
> Subject: Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic
> decode
> 
> * Babu Moger (babu.moger@amd.com) wrote:
> >
> > > -----Original Message-----
> > > From: Igor Mammedov <imammedo@redhat.com>
> > > Sent: Wednesday, August 26, 2020 8:31 AM
> > > To: Daniel P. Berrangé <berrange@redhat.com>
> > > Cc: Moger, Babu <Babu.Moger@amd.com>; pbonzini@redhat.com;
> > > rth@twiddle.net; ehabkost@redhat.com; qemu-devel@nongnu.org;
> > > mst@redhat.com; Michal Privoznik <mprivozn@redhat.com>
> > > Subject: Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use
> > > generic decode
> > >
> > > 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.
> > > > > > https://nam11.safelinks.protection.outlook.com/?url=https%3A%2
> > > > > > F%2F
> > > > > > lore.kernel.org%2Fqemu-devel%2Fc0bcc1a6-1d84-a6e7-e468-
> > > d5b437c1b25
> > > > > >
> > >
> 4%40amd.com%2F&amp;data=02%7C01%7Cbabu.moger%40amd.com%7C8a5c
> > > 52f92
> > > > > >
> > >
> 3f04082a40808d849c43d49%7C3dd8961fe4884e608e11a82d994e183d%7C0%7
> > > C0
> > > > > >
> > >
> %7C637340454473508873&amp;sdata=VnW28H1v4XwK3GaNGFxu%2BhwiMeA
> > > YO%2B
> > > > > > 3WAzo3DeY5Ha8%3D&amp;reserved=0
> > > > > >
> > > > > > 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
> > No, That is not true. Because of that assumption we made all these
> > apicid changes. And here we are now.
> >
> > AMD supports varies mixed configurations. In case of EPYC-Rome, we
> > have NPS1, NPS2 and NPS4(Numa Nodes per socket). In case of NPS1,
> > basically we have all the cores in a socket under one numa node. This
> > is non-numa configuration.
> > Looking at the various configurations and also discussing internally,
> > it is not advisable to have (epyc && !numa) check.
> 
> Indeed on real hardware, I don't think we always see NUMA; my single socket,
> 16 core/32 thread 7302P Dell box, shows the kernel printing 'No NUMA
> configuration found...Faking a node.'
> 
> So if real hardware hasn't got a NUMA node, what's the real problem?

I don't see any problem once we revert all these changes(patch 1-7).
We don't need if (epyc && !numa) error check or auto_enable_numa=true
unconditionally.

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