Commit a3abd0f2 authored by Igor Mammedov's avatar Igor Mammedov Committed by Eduardo Habkost
Browse files

pc: Clarify FW_CFG_MAX_CPUS usage comment 



Signed-off-by: default avatarIgor Mammedov <imammedo@redhat.com>
Reviewed-by: default avatarEduardo Habkost <ehabkost@redhat.com>
Signed-off-by: default avatarEduardo Habkost <ehabkost@redhat.com>
parent e391c009

hw/i386/pc.c

+8 −10
Original line number Diff line number Diff line
@@ -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);