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authorIgor Mammedov <imammedo@redhat.com>2016-10-19 14:05:39 +0200
committerEduardo Habkost <ehabkost@redhat.com>2016-10-24 17:29:15 -0200
commita3abd0f28eb863206b4466a5bb484eb0d4025819 (patch)
treef00a4cefc7912ce954bab20d54f7cf259bb7bd6d /hw
parente391c00970a3fe3448b78054fb917df751847b16 (diff)
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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>
Diffstat (limited to 'hw')
-rw-r--r--hw/i386/pc.c18
1 files changed, 8 insertions, 10 deletions
diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index b1f8638..83ad556 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);