Re: [PATCH v7 07/13] hw/386: Add EPYC mode topology decoding functions

[Eduardo Habkost]

On Wed, Jun 03, 2020 at 10:22:10AM -0500, Babu Moger wrote:
> 
> 
> > -----Original Message-----
> > From: Eduardo Habkost <ehabkost@redhat.com>
> > Sent: Tuesday, June 2, 2020 6:01 PM
> > To: Moger, Babu <Babu.Moger@amd.com>
> > Cc: marcel.apfelbaum@gmail.com; pbonzini@redhat.com; rth@twiddle.net;
> > mst@redhat.com; imammedo@redhat.com; qemu-devel@nongnu.org
> > Subject: Re: [PATCH v7 07/13] hw/386: Add EPYC mode topology decoding
> > functions
> > 
> > On Tue, Jun 02, 2020 at 04:59:19PM -0500, Babu Moger wrote:
> > >
> > >
> > > > -----Original Message-----
> > > > From: Eduardo Habkost <ehabkost@redhat.com>
> > > > Sent: Tuesday, June 2, 2020 12:19 PM
> > > > To: Moger, Babu <Babu.Moger@amd.com>
> > > > Cc: marcel.apfelbaum@gmail.com; pbonzini@redhat.com; rth@twiddle.net;
> > > > mst@redhat.com; imammedo@redhat.com; qemu-devel@nongnu.org
> > > > Subject: Re: [PATCH v7 07/13] hw/386: Add EPYC mode topology decoding
> > > > functions
> > > >
> > > > Hi,
> > > >
> > > > It looks like this series breaks -device and CPU hotplug:
> > > >
> > > > On Wed, Mar 11, 2020 at 05:53:34PM -0500, Babu Moger wrote:
> > > > > These functions add support for building EPYC mode topology given the
> > smp
> > > > > details like numa nodes, cores, threads and sockets.
> > > > >
> > > > > The new apic id decoding is mostly similar to current apic id decoding
> > > > > except that it adds a new field node_id when numa configured. Removes
> > all
> > > > > the hardcoded values. Subsequent patches will use these functions to 
> > > > > build
> > > > > the topology.
> > > > >
> > > > > Following functions are added.
> > > > > apicid_llc_width_epyc
> > > > > apicid_llc_offset_epyc
> > > > > apicid_pkg_offset_epyc
> > > > > apicid_from_topo_ids_epyc
> > > > > x86_topo_ids_from_idx_epyc
> > > > > x86_topo_ids_from_apicid_epyc
> > > > > x86_apicid_from_cpu_idx_epyc
> > > > >
> > > > > The topology details are available in Processor Programming Reference
> > (PPR)
> > > > > for AMD Family 17h Model 01h, Revision B1 Processors. The revision
> > guides
> > > > are
> > > > > available from the bugzilla Link below.
> > > > > Link:
> > > >
> > https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugzilla.
> > > >
> > kernel.org%2Fshow_bug.cgi%3Fid%3D206537&amp;data=02%7C01%7Cbabu.m
> > > >
> > oger%40amd.com%7C3487f40d37df4d59097d08d807190248%7C3dd8961fe488
> > > >
> > 4e608e11a82d994e183d%7C0%7C0%7C637267151289763739&amp;sdata=wE0
> > > > ukXIVh0l5eNQWsv6VDE9UZEVJmisofaW192gcZAI%3D&amp;reserved=0
> > > > >
> > > > > Signed-off-by: Babu Moger <babu.moger@amd.com>
> > > > > Acked-by: Igor Mammedov <imammedo@redhat.com>
> > > > > Acked-by: Michael S. Tsirkin <mst@redhat.com>
> > > > > ---
> > > > [...]
> > > > >  typedef struct X86CPUTopoIDs {
> > > > >      unsigned pkg_id;
> > > > > +    unsigned node_id;
> > > >
> > > > You have added a new field here.
> > > >
> > > > >      unsigned die_id;
> > > > >      unsigned core_id;
> > > > >      unsigned smt_id;
> > > > [...]
> > > > > +static inline apic_id_t
> > > > > +x86_apicid_from_topo_ids_epyc(X86CPUTopoInfo *topo_info,
> > > > > +                              const X86CPUTopoIDs *topo_ids)
> > > > > +{
> > > > > +    return (topo_ids->pkg_id  << apicid_pkg_offset_epyc(topo_info)) |
> > > > > +           (topo_ids->node_id << apicid_node_offset_epyc(topo_info)) 
> > > > > |
> > > >
> > > > You are using the new field here.
> > > >
> > > > > +           (topo_ids->die_id  << apicid_die_offset(topo_info)) |
> > > > > +           (topo_ids->core_id << apicid_core_offset(topo_info)) |
> > > > > +           topo_ids->smt_id;
> > > > > +}
> > > >
> > > > But you are not initializing node_id in one caller of 
> > > > apicid_from_topo_ids():
> > > >
> > > > static void pc_cpu_pre_plug(HotplugHandler *hotplug_dev,
> > > >                             DeviceState *dev, Error **errp)
> > > > {
> > > >     [...]
> > > >     X86CPUTopoIDs topo_ids;
> > > >     [...]
> > > >     if (cpu->apic_id == UNASSIGNED_APIC_ID) {
> > > >         [...]
> > > >         topo_ids.pkg_id = cpu->socket_id;
> > > >         topo_ids.die_id = cpu->die_id;
> > > >         topo_ids.core_id = cpu->core_id;
> > > >         topo_ids.smt_id = cpu->thread_id;
> > > >         cpu->apic_id = x86ms->apicid_from_topo_ids(&topo_info, 
> > > > &topo_ids);
> > > >     }
> > > >     [...]
> > > > }
> > > >
> > > > Result: -device is broken when using -cpu EPYC:
> > > >
> > > >   $ 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
> > 
> > [1]
> > 
> > > >   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 patch should initialize the node_id. But I am not sure how to
> > > reproduce the bug. Can you please send me the full command line to
> > > reproduce the problem. Also test different options.
> > 
> > The full command line is above[1].
> 
> I just picked up the latest tree from
> git clone https://git.qemu.org/git/qemu.git qemu
> Not seeing the problem.
> 
> ./x86_64-softmmu/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
> VNC server running on ::1:5900
> 
> It appears to run ok.
> 
> > 
> > 
> > >
> > > diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> > > index 2128f3d6fe..047b4b9391 100644
> > > --- a/hw/i386/pc.c
> > > +++ b/hw/i386/pc.c
> > > @@ -1539,6 +1539,9 @@ static void pc_cpu_pre_plug(HotplugHandler
> > *hotplug_dev,
> > >      if (cpu->apic_id == UNASSIGNED_APIC_ID) {
> > >          int max_socket = (ms->smp.max_cpus - 1) /
> > >                                  smp_threads / smp_cores / 
> > > x86ms->smp_dies;
> > 
> > So, here's the input you are using to calculate topo_ids.node_id:
> > 
> > > +        unsigned nr_nodes = MAX(topo_info.nodes_per_pkg, 1);
> > 
> > When is topo_info.nodes_per_pkg allowed to be 0?
> 
> It will be zero if numa is not configured. Node_id will be zero for all
> the cores.
> 
> > 
> > > +        unsigned cores_per_node = DIV_ROUND_UP((x86ms->smp_dies *
> > smp_cores *
> > > +                                                smp_threads), nr_nodes);
> > 
> > x86ms->smp_dies should be available at topo_info.dies_per_pkg,
> > smp_cores should available at topo_info.cores_per_die,
> > smp_threads should be available at topo_info.threads_per_core,
> > nr_nodes should be available at topo_info.nodes_per_pkg.
> > 
> > >
> > >          /*
> > >           * die-id was optional in QEMU 4.0 and older, so keep it optional
> > > @@ -1585,6 +1588,7 @@ 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->core_id / cores_per_node) % nr_nodes;
> > 
> > apicid_from_topo_ids() have access to topo_info and topo_ids, If
> > all the information you need to calculate node_id is already
> > available inside topo_info + topo_ids, we could be calculating it
> > inside apicid_from_topo_ids().  Why don't we do it?
> > 
> > Also, is topo_ids.core_id really allowed to be larger than
> > cores_per_node when calling apicid_from_topo_ids()?
> 
> Yes. It is. If we have two numa nodes and 8 cores. Then cores_per_node is
> 4. Nr_nodes =2. Yes. Core_id can be larger than cores_per_node.

I assumed core_id identified the core inside a die, and the range
would be [0, cores_per_die).  This seems to be what
apicid_from_topo_ids_epyc() expects.

We need clearer documentation on the semantics of each *_id
field, to avoid confusion.

> 
> > 
> > >          cpu->apic_id =
> > >          x86ms->apicid_from_topo_ids(&topo_info,
> > >          &topo_ids); }
> > >
> > 
> > -- Eduardo
> 

-- 
Eduardo