RE: [PATCH 1/2] hw/386: Fix uninitialized memory with -device and CPU hotplug

[Babu Moger <babu.moger@amd.com>]

> -----Original Message-----
> From: Igor Mammedov <imammedo@redhat.com>
> Sent: Wednesday, June 24, 2020 8:48 AM
> To: Moger, Babu <Babu.Moger@amd.com>
> Cc: ehabkost@redhat.com; mst@redhat.com; qemu-devel@nongnu.org;
> pbonzini@redhat.com; rth@twiddle.net
> Subject: Re: [PATCH 1/2] hw/386: Fix uninitialized memory with -device and CPU
> hotplug
> 
> On Tue, 16 Jun 2020 12:18:56 -0500
> Babu Moger <babu.moger@amd.com> wrote:
> 
> > > -----Original Message-----
> > > From: Igor Mammedov <imammedo@redhat.com>
> > > Sent: Tuesday, June 16, 2020 5:59 AM
> > > To: Moger, Babu <Babu.Moger@amd.com>
> > > Cc: pbonzini@redhat.com; rth@twiddle.net; ehabkost@redhat.com;
> > > mst@redhat.com; marcel.apfelbaum@gmail.com; qemu-devel@nongnu.org
> > > Subject: Re: [PATCH 1/2] hw/386: Fix uninitialized memory with -device and
> CPU
> > > hotplug
> > >
> > > On Mon, 08 Jun 2020 15:18:50 -0500
> > > Babu Moger <babu.moger@amd.com> wrote:
> > >
> > > > Noticed the following command failure while testing CPU hotplug.
> > > >
> > > > $ qemu-system-x86_64 -machine q35,accel=kvm -smp 1,maxcpus=2,
> > > >   cores=1, threads=1,sockets=2 -cpu EPYC -device EPYC-x86_64-
> > > >   cpu,core-id=0,socket-id=1,thread-id=0
> > > >
> > > >   qemu-system-x86_64: -device EPYC-x86_64-cpu,core-id=0,socket-id=1,
> > > >   thread-id=0: Invalid CPU [socket: 21855, die: 0, core: 0, thread: 0]
> > > >   with APIC ID 21855, valid index range 0:1
> > > >
> > > > This happens because APIC ID is calculated using uninitialized memory.
> > > > This is happening after the addition of new field node_id in
> X86CPUTopoIDs
> > > > structure. The node_id field is uninitialized while calling
> > > > apicid_from_topo_ids. The problem is discussed in the thread below.
> > > >
> > >
> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.ker
> > > nel.org%2Fqemu-
> > >
> devel%2F20200602171838.GG577771%40habkost.net%2F&amp;data=02%7C01
> > >
> %7Cbabu.moger%40amd.com%7C02200d75fd8b48d1955608d811e44f5b%7C3d
> > >
> d8961fe4884e608e11a82d994e183d%7C0%7C0%7C637279019564311233&amp
> > >
> ;sdata=ry3QO0Z5dxLPoRxkYVkOsVm3nl%2BxfCGv8be%2BMHdoUPY%3D&amp;r
> > > eserved=0
> > > >
> > > > Fix the problem by initializing the node_id properly.
> > > >
> > > > Signed-off-by: Babu Moger <babu.moger@amd.com>
> > > > ---
> > > >  hw/i386/pc.c               |    2 ++
> > > >  include/hw/i386/topology.h |   11 +++++++++++
> > > >  2 files changed, 13 insertions(+)
> > > >
> > > > diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> > > > index 2128f3d6fe..974cc30891 100644
> > > > --- a/hw/i386/pc.c
> > > > +++ b/hw/i386/pc.c
> > > > @@ -1585,6 +1585,8 @@ static void pc_cpu_pre_plug(HotplugHandler
> > > *hotplug_dev,
> > > >          topo_ids.die_id = cpu->die_id;
> > > >          topo_ids.core_id = cpu->core_id;
> > > >          topo_ids.smt_id = cpu->thread_id;
> > > > +        topo_ids.node_id = cpu_x86_use_epyc_apic_id_encoding(ms-
> >cpu_type)
> > > ?
> > > > +                           x86_node_id_for_epyc(&topo_info, &topo_ids) : 0;
> > >
> > > I'd rather not calculate some default value here,
> > > this is the branch where we check user provided topology info and error out
> > > asking
> > > to provide missing bits.
> > Noticed that cpu->node_id is initialized to 0xFF(NUMA_NODE_UNASSIGNED).
> > We can initialize cpu->node_id to default node like how we do it in
> > x86_get_default_cpu_node_id.  We can use it to initialize topo_ids.node_id.
> > This is consistent with other fields core_id, die_id etc..
> >
> > >
> > > I also wonder if we should force user to specify numa nodes on CLI if EPYC
> cpu is
> > > used.
> > > (i.e. I'm assuming that EPYC always requires numa)
> >
> > That is not true. Without numa all the cpus will be configured under one
> > default numa node 0. Like we do it using x86_get_default_cpu_node_id.
> 
> get_default_cpu_node_id() which is making things up, is going to be removed
> eventually in favor of asking user to provide numa mapping explicitly on CLI.

That will be good going forward.

> 
> now if it's always only node 0, why do we need to calculate it then,
> why not just assing 0 directly?
> 
> what if we have several sockets, would all vCPUs still have node-id = 0?

If there are several nodes then socket id becomes node id.

> 
> Another question is if we have CPUs with only 1 numa node set on all of them,
> does it require all other machinery like ACPI SRAT table defined or it's just
> internal CPU impl. detail?

I am not very sure about it. To me it looks like it is just internal cpu
implementation detail.

I think we have two options here.

1. Update the patch to initialize the node_id the way it is done
get_default_cpu_node_id.
2. Ask the user to pass the node_id while hot plugging the device. This is
a clean solution. Will require some data structure changes.

Let me know if you see any other option.

> 
> 
> > >
> > > >          cpu->apic_id = x86ms->apicid_from_topo_ids(&topo_info,
> &topo_ids);
> > > >      }
> > > >
> > > > diff --git a/include/hw/i386/topology.h b/include/hw/i386/topology.h
> > > > index 07239f95f4..ee4deb84c4 100644
> > > > --- a/include/hw/i386/topology.h
> > > > +++ b/include/hw/i386/topology.h
> > > > @@ -140,6 +140,17 @@ static inline unsigned
> > > apicid_pkg_offset_epyc(X86CPUTopoInfo *topo_info)
> > > >             apicid_node_width_epyc(topo_info);
> > > >  }
> > > >
> > > > +static inline unsigned x86_node_id_for_epyc(X86CPUTopoInfo
> *topo_info,
> > > > +                                            const X86CPUTopoIDs *topo_ids)
> > > > +{
> > > > +    unsigned nr_nodes = MAX(topo_info->nodes_per_pkg, 1);
> > > > +    unsigned cores_per_node = DIV_ROUND_UP((topo_info->dies_per_pkg
> *
> > > > +                                            topo_info->cores_per_die *
> > > > +                                            topo_info->threads_per_core),
> > > > +                                            nr_nodes);
> > > > +
> > > > +    return (topo_ids->core_id / cores_per_node) % nr_nodes;
> > > what if nr_nodes == 0?
> > >
> > > > +}
> > > >  /*
> > > >   * Make APIC ID for the CPU based on Pkg_ID, Core_ID, SMT_ID
> > > >   *
> > > >
> > > >
> >
> >