Re: [PATCH 3/4] hw/i386: add facility to expose CPU topology over fw-cfg

[Igor Mammedov <imammedo@redhat.com>]

On Tue,  8 Oct 2019 12:52:58 +0200
Laszlo Ersek <lersek@redhat.com> wrote:

> FW_CFG_MAX_CPUS exposes the (exclusive) maximum APIC ID to guest firmware,
> due to historical reasons. That value is not useful to edk2, however. For
> supporting VCPU hotplug, edk2 needs:
> 
> - the boot CPU count (already exposed in FW_CFG_NB_CPUS),
> 
> - and the maximum foreseen CPU count (tracked in
>   "MachineState.smp.max_cpus", but not currently exposed).
one can get it with current QEMU without adding new fgcfg
(albeit in a bit awkward way)

max_cpu count can be derived indirectly as result of cpu hotplug
enumeration (IO interface at 0x0cd8-0xcf7) by writing/reading
to/from selector register (see ACPI_CPU_SELECTOR_OFFSET_WR)
until read value stops changing values (i.e. max cpu count
is reached). One also can figure out present/non-present
cpu status by reading flags register.

> Add a new fw-cfg file to expose "max_cpus".
> 
> While at it, expose the rest of the topology too (die / core / thread
> counts), because I expect that the VCPU hotplug feature for OVMF will
> ultimately need those too, and the data size is not large. This is
> slightly complicated by the fact that the die count is specific to
> PCMachineState, but fw_cfg_arch_create() intends to be PC-independent (see
> commit 149c50cabcc4).
Could you clarify why topology info is necessary?

Potentially it's possible to extend cpu hotplug ABI to report
arch specific apic-id (x86) or mpidr (arm) if firmware really
needs to know topology and let guest to decode it according
to CPU's spec.

So far there were no need for it as all possible cpus are
described in ACPI tables passed to guest, but I'm not going
to suggest to parse them on firmware side as it's too complicated :)

PS:
The reason we started building ACPI tables in QEMU, was never
ending story of adding more ABI and supporting it afterwards.
So I'd try to avoid doing it if it can be helped.

> For now, the feature is temporarily disabled.
> 
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Eduardo Habkost <ehabkost@redhat.com>
> Cc: Igor Mammedov <imammedo@redhat.com>
> Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
> Cc: Richard Henderson <rth@twiddle.net>
> Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1515
> Signed-off-by: Laszlo Ersek <lersek@redhat.com>
> ---
>  hw/i386/fw_cfg.h | 30 +++++++++++++++++++++++++++++-
>  hw/i386/fw_cfg.c | 26 ++++++++++++++++++++++++--
>  hw/i386/pc.c     |  4 ++--
>  3 files changed, 55 insertions(+), 5 deletions(-)
> 
> diff --git a/hw/i386/fw_cfg.h b/hw/i386/fw_cfg.h
> index e0856a376996..d742435b9793 100644
> --- a/hw/i386/fw_cfg.h
> +++ b/hw/i386/fw_cfg.h
> @@ -18,9 +18,37 @@
>  #define FW_CFG_E820_TABLE       (FW_CFG_ARCH_LOCAL + 3)
>  #define FW_CFG_HPET             (FW_CFG_ARCH_LOCAL + 4)
>  
> +/**
> + * FWCfgX86Topology: expose the X86 CPU topology to guest firmware over fw-cfg.
> + *
> + * All fields have little-endian encoding.
> + *
> + * @dies:     Number of dies per package (aka socket). Set it to 1 unless the
> + *            concrete MachineState subclass defines it differently.
> + * @cores:    Corresponds to @CpuTopology.@cores.
> + * @threads:  Corresponds to @CpuTopology.@threads.
> + * @max_cpus: Corresponds to @CpuTopology.@max_cpus.
> + *
> + * Firmware can derive the package (aka socket) count with the following
> + * formula:
> + *
> + *   DIV_ROUND_UP(@max_cpus, @dies * @cores * @threads)
> + *
> + * Firmware can derive APIC ID field widths and offsets per the standard
> + * calculations in "include/hw/i386/topology.h".
> + */
> +typedef struct FWCfgX86Topology {
> +  uint32_t dies;
> +  uint32_t cores;
> +  uint32_t threads;
> +  uint32_t max_cpus;
> +} QEMU_PACKED FWCfgX86Topology;
> +
>  FWCfgState *fw_cfg_arch_create(MachineState *ms,
>                                 uint16_t boot_cpus,
> -                               uint16_t apic_id_limit);
> +                               uint16_t apic_id_limit,
> +                               unsigned smp_dies,
> +                               bool expose_topology);
>  void fw_cfg_build_smbios(MachineState *ms, FWCfgState *fw_cfg);
>  void fw_cfg_build_feature_control(MachineState *ms, FWCfgState *fw_cfg);
>  
> diff --git a/hw/i386/fw_cfg.c b/hw/i386/fw_cfg.c
> index 39b6bc60520c..33d09875014f 100644
> --- a/hw/i386/fw_cfg.c
> +++ b/hw/i386/fw_cfg.c
> @@ -85,9 +85,26 @@ void fw_cfg_build_smbios(MachineState *ms, FWCfgState *fw_cfg)
>      }
>  }
>  
> +static void fw_cfg_expose_topology(FWCfgState *fw_cfg,
> +                                   unsigned dies,
> +                                   unsigned cores,
> +                                   unsigned threads,
> +                                   unsigned max_cpus)
> +{
> +    FWCfgX86Topology *topo = g_new(FWCfgX86Topology, 1);
> +
> +    topo->dies     = cpu_to_le32(dies);
> +    topo->cores    = cpu_to_le32(cores);
> +    topo->threads  = cpu_to_le32(threads);
> +    topo->max_cpus = cpu_to_le32(max_cpus);
> +    fw_cfg_add_file(fw_cfg, "etc/x86-smp-topology", topo, sizeof *topo);
> +}
> +
>  FWCfgState *fw_cfg_arch_create(MachineState *ms,
> -                                      uint16_t boot_cpus,
> -                                      uint16_t apic_id_limit)
> +                               uint16_t boot_cpus,
> +                               uint16_t apic_id_limit,
> +                               unsigned smp_dies,
> +                               bool expose_topology)
>  {
>      FWCfgState *fw_cfg;
>      uint64_t *numa_fw_cfg;
> @@ -143,6 +160,11 @@ FWCfgState *fw_cfg_arch_create(MachineState *ms,
>                       (1 + apic_id_limit + nb_numa_nodes) *
>                       sizeof(*numa_fw_cfg));
>  
> +    if (expose_topology) {
> +        fw_cfg_expose_topology(fw_cfg, smp_dies, ms->smp.cores,
> +                               ms->smp.threads, ms->smp.max_cpus);
> +    }
> +
>      return fw_cfg;
>  }
>  
> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> index bcda50efcc23..bb72b12edad2 100644
> --- a/hw/i386/pc.c
> +++ b/hw/i386/pc.c
> @@ -1738,8 +1738,8 @@ void pc_memory_init(PCMachineState *pcms,
>                                          option_rom_mr,
>                                          1);
>  
> -    fw_cfg = fw_cfg_arch_create(machine,
> -                                pcms->boot_cpus, pcms->apic_id_limit);
> +    fw_cfg = fw_cfg_arch_create(machine, pcms->boot_cpus, pcms->apic_id_limit,
> +                                pcms->smp_dies, false);
>  
>      rom_set_fw(fw_cfg);
>