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

[Igor Mammedov <imammedo@redhat.com>]

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://lore.kernel.org/qemu-devel/20200602171838.GG577771@habkost.net/
> 
> 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.

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)

>          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
>   *
> 
>