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

[Daniel P . Berrangé]

On Thu, Aug 27, 2020 at 10:21:10PM +0200, Igor Mammedov wrote:
> On Wed, 26 Aug 2020 13:45:51 -0500
> Babu Moger <babu.moger@amd.com> wrote:
> 
> > 
> > 
> > > -----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.'
> looks like firmware bug or maybe it's feature and there is a knob in fw
> to turn it on/off in case used OS doesn't like it for some reason.
> 
> 
> > > 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.
> 
> We need revert to unbreak migration from QEMU < 5.0,
> everything else (fixes for CPUID_Fn8000001E) could go on top.
> 
> So what's on top (because old code also wasn't correct when
> CPUID_Fn8000001E is taken in account, tha's why we are at this point),
> 
> When starting QEMU without -numa
> Indeed we can skip "if (epyc && !numa) error check or auto_enable_numa=true",
> in case where there is 1 die (NPS1).
> (1) User however may set core/threads number bigger than possible by spec,
>     in which case CPUID_Fn8000001E_EBX/CPUID_Fn8000001E_ECX will not be
>     valid spec vise and could trigger OPPs in guest kernel.
>     Given we allow go out of spec, perhaps we should add a warning at
>     realize time saying that used -smp config is not supported since it
>     doesn't match AMD EPYC spec and might not work.
> 
> (2) Earlier we agreed that we can reuse existing die_id instead of internal
>     (topo_ids.node_id in current code)
>     (It's is called DIE_ID and NODE ID in spec interchangeably)
>     Same as (1) add a warning when '-smp dies' goes beyond spec limits.
>     
> (3) "-smp dies>1" ''if'' we allow to run it without -numa,
>     then system wide NUMA node id in CPUID_Fn8000001E_ECX probably doesn't 
> matter.
>     could be something like in spec but taking in account die offset, to 
> produce
>     unique id.
> 
>     Same, add a warning that there are more than 1 dies but numa is not 
> enabled,
>     suggest to enable numa.
> 
>     With current code it produces invalid APIC ID for valid '-smp' 
> combination,
>     however if we revert it and switch to die_id than it should produce
>     valid APIC ID once again (as in 4.2).
>     Given it produces invalid APIC id, maybe we should just ditch the case and
>     fold it in (4) (i.e. require -numa if "-smp dies>1")
> 
> (4) -numa is used (RHBZ1728166)
>     we need to ensure that socket*dies == ms->numa_state->num_nodes
>      and make sure that CPUID_Fn8000001E_ECX consistent with
>     cpu mapping provided with "-numa cpu=" option.

Why do we need to socket*dies == ms->numa_state->num_nodes ? That doesn't
seem to be the case in bare metal EPYC nodes I've used which lets you
configure how many NUMA nodes in firmware.


Regards,
Daniel
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