diff options
| author | Igor Mammedov <imammedo@redhat.com> | 2016-10-19 14:05:39 +0200 |
|---|---|---|
| committer | Eduardo Habkost <ehabkost@redhat.com> | 2016-10-24 17:29:15 -0200 |
| commit | a3abd0f28eb863206b4466a5bb484eb0d4025819 (patch) | |
| tree | f00a4cefc7912ce954bab20d54f7cf259bb7bd6d /hw | |
| parent | e391c00970a3fe3448b78054fb917df751847b16 (diff) | |
pc: Clarify FW_CFG_MAX_CPUS usage comment
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Diffstat (limited to 'hw')
| -rw-r--r-- | hw/i386/pc.c | 18 |
1 files changed, 8 insertions, 10 deletions
diff --git a/hw/i386/pc.c b/hw/i386/pc.c index b1f863807b..83ad556657 100644 --- a/hw/i386/pc.c +++ b/hw/i386/pc.c @@ -746,17 +746,15 @@ static FWCfgState *bochs_bios_init(AddressSpace *as, PCMachineState *pcms) /* 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". + * For machine types prior to 1.8, SeaBIOS needs FW_CFG_MAX_CPUS for + * building MPTable, ACPI MADT, ACPI CPU hotplug and ACPI SRAT table, + * that tables are based on xAPIC ID and QEMU<->SeaBIOS interface + * for CPU hotplug also uses APIC ID and not "CPU index". + * 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" + * So for compatibility reasons with old BIOSes we are stuck with + * "etc/max-cpus" actually being apic_id_limit */ fw_cfg_add_i16(fw_cfg, FW_CFG_MAX_CPUS, (uint16_t)pcms->apic_id_limit); fw_cfg_add_i64(fw_cfg, FW_CFG_RAM_SIZE, (uint64_t)ram_size); |