Re: [Qemu-devel] [PATCH for-1.4 qom-cpu 6/9] pc: Set fw_cfg data based on APIC ID calculation

[Andreas Färber]

Am 22.01.2013 21:25, schrieb Eduardo Habkost:
> This changes FW_CFG_MAX_CPUS and FW_CFG_NUMA to use apic_id_for_cpu(),
> so the NUMA table can be based on the APIC IDs, instead of CPU index
> (SeaBIOS knows nothing about CPU indexes, just APIC IDs).
> 
> Signed-off-by: Eduardo Habkost <address@hidden>
> ---
> Changes v2:
>  - Get PC object as argument
>  - Add more detailed comments explaining the reason for FW_CFG_MAX_CPUS
>    not being simply 'max_cpus'
> 
> Changes v3:
>  - Use PCInitArgs instead of PC object
> 
> Changes v4:
>  - Don't use PCInitArgs, just add the necessary data for apic_id_limit()
>    as argument
>  - Rename function to pc_apic_id_limit()
>  - Rename max_apic_id to apic_id_limit
> 
> Changes v5:
>  - Refresh after apic_id_for_cpu() -> x86_cpu_apic_id_from_index()
>    rename
>  - Refresh after original code changes to use g_new0()
> ---
>  hw/pc.c | 45 ++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 38 insertions(+), 7 deletions(-)
> 
> diff --git a/hw/pc.c b/hw/pc.c
> index 44bb1dc..9029a55 100644
> --- a/hw/pc.c
> +++ b/hw/pc.c
> @@ -551,6 +551,18 @@ int e820_add_entry(uint64_t address, uint64_t length, 
> uint32_t type)
>      return index;
>  }
>  
> +/* Calculates the limit to CPU APIC ID values
> + *
> + * This function returns the limit for the APIC ID value, so that all
> + * CPU APIC IDs are < pc_apic_id_limit().
> + *
> + * This is used for FW_CFG_MAX_CPUS. See comments on bochs_bios_init().
> + */
> +static unsigned int pc_apic_id_limit(unsigned int max_cpus)
> +{
> +    return x86_cpu_apic_id_from_index(max_cpus - 1) + 1;
> +}
> +
>  static void *bochs_bios_init(void)
>  {
>      void *fw_cfg;
> @@ -558,9 +570,24 @@ static void *bochs_bios_init(void)
>      size_t smbios_len;
>      uint64_t *numa_fw_cfg;
>      int i, j;
> +    unsigned int apic_id_limit = pc_apic_id_limit(max_cpus);
>  
>      fw_cfg = fw_cfg_init(BIOS_CFG_IOPORT, BIOS_CFG_IOPORT + 1, 0, 0);
> -    fw_cfg_add_i16(fw_cfg, FW_CFG_MAX_CPUS, (uint16_t)max_cpus);
> +    /* FW_CFG_MAX_CPUS is a bit confusing/problematic on x86:
> +     *
> +     * SeaBIOS needs FW_CFG_MAX_CPUS for CPU hotplug, but the CPU hotplug
> +     * QEMU<->SeaBIOS interface is not based on the "CPU index", but on the 
> APIC
> +     * ID of hotplugged CPUs[1]. This means that FW_CFG_MAX_CPUS is not the
> +     * "maximum number of CPUs", but the "limit to the APIC ID values SeaBIOS
> +     * may see".
> +     *
> +     * So, this means we must not use max_cpus, here, but the maximum 
> possible
> +     * APIC ID value, plus one.
> +     *
> +     * [1] The only kind of "CPU identifier" used between SeaBIOS and QEMU is
> +     *     the APIC ID, not the "CPU index"
> +     */
> +    fw_cfg_add_i16(fw_cfg, FW_CFG_MAX_CPUS, (uint16_t)apic_id_limit);
>      fw_cfg_add_i32(fw_cfg, FW_CFG_ID, 1);
>      fw_cfg_add_i64(fw_cfg, FW_CFG_RAM_SIZE, (uint64_t)ram_size);
>      fw_cfg_add_bytes(fw_cfg, FW_CFG_ACPI_TABLES,
> @@ -579,21 +606,25 @@ static void *bochs_bios_init(void)
>       * of nodes, one word for each VCPU->node and one word for each node to
>       * hold the amount of memory.
>       */
> -    numa_fw_cfg = g_new0(uint64_t, 1 + max_cpus + nb_numa_nodes);
> +    numa_fw_cfg = g_new0(uint64_t, 1 + apic_id_limit + nb_numa_nodes);
>      numa_fw_cfg[0] = cpu_to_le64(nb_numa_nodes);
> -    for (i = 0; i < max_cpus; i++) {
> +    unsigned int cpu_idx;

Beep.

> +    for (cpu_idx = 0; cpu_idx < max_cpus; cpu_idx++) {
> +        unsigned int apic_id = x86_cpu_apic_id_from_index(cpu_idx);
> +        assert(apic_id < apic_id_limit);
>          for (j = 0; j < nb_numa_nodes; j++) {
> -            if (test_bit(i, node_cpumask[j])) {
> -                numa_fw_cfg[i + 1] = cpu_to_le64(j);
> +            if (test_bit(cpu_idx, node_cpumask[j])) {
> +                numa_fw_cfg[apic_id + 1] = cpu_to_le64(j);
>                  break;
>              }
>          }
>      }

Why can't we keep using i here? That would leave the "for (..." and
"test_bit" lines unchanged and let us spot the actual changes of i vs.
apic_id more easily.

Andreas

>      for (i = 0; i < nb_numa_nodes; i++) {
> -        numa_fw_cfg[max_cpus + 1 + i] = cpu_to_le64(node_mem[i]);
> +        numa_fw_cfg[apic_id_limit + 1 + i] = cpu_to_le64(node_mem[i]);
>      }
>      fw_cfg_add_bytes(fw_cfg, FW_CFG_NUMA, numa_fw_cfg,
> -                     (1 + max_cpus + nb_numa_nodes) * sizeof(*numa_fw_cfg));
> +                     (1 + apic_id_limit + nb_numa_nodes) *
> +                     sizeof(*numa_fw_cfg));
>  
>      return fw_cfg;
>  }
> 


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