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>

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);