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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>
This commit is contained in:
Igor Mammedov 2016-10-19 14:05:39 +02:00 committed by Eduardo Habkost
parent e391c00970
commit a3abd0f28e
1 changed files with 8 additions and 10 deletions
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18
hw/i386/pc.c
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@ -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);