On Wed, 3 Dec 2025 00:28:13 +0800 Zhao Liu <zhao1.liu@intel.com> wrote:
Legacy cpu hotplug has been removed totally and machines start with modern cpu hotplug interface directly.
Therefore, update the documentation to describe current QEMU cpu hotplug logic.
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
--- Changes since v4: * New patch. --- docs/specs/acpi_cpu_hotplug.rst | 28 +++------------------------- 1 file changed, 3 insertions(+), 25 deletions(-)
diff --git a/docs/specs/acpi_cpu_hotplug.rst b/docs/specs/acpi_cpu_hotplug.rst index 351057c96761..f49678100044 100644 --- a/docs/specs/acpi_cpu_hotplug.rst +++ b/docs/specs/acpi_cpu_hotplug.rst @@ -8,22 +8,6 @@ ACPI BIOS GPE.2 handler is dedicated for notifying OS about CPU hot-add and hot-remove events.
-Legacy ACPI CPU hotplug interface registers -------------------------------------------- - -CPU present bitmap for: - -- ICH9-LPC (IO port 0x0cd8-0xcf7, 1-byte access) -- PIIX-PM (IO port 0xaf00-0xaf1f, 1-byte access) -- One bit per CPU. Bit position reflects corresponding CPU APIC ID. Read-only. -- The first DWORD in bitmap is used in write mode to switch from legacy - to modern CPU hotplug interface, write 0 into it to do switch. - -QEMU sets corresponding CPU bit on hot-add event and issues SCI -with GPE.2 event set. CPU present map is read by ACPI BIOS GPE.2 handler -to notify OS about CPU hot-add events. CPU hot-remove isn't supported. - - Modern ACPI CPU hotplug interface registers -------------------------------------------
@@ -189,20 +173,14 @@ Typical usecases (x86) Detecting and enabling modern CPU hotplug interface ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-QEMU starts with legacy CPU hotplug interface enabled. Detecting and -switching to modern interface is based on the 2 legacy CPU hotplug features: - -#. Writes into CPU bitmap are ignored. -#. CPU bitmap always has bit #0 set, corresponding to boot CPU. - -Use following steps to detect and enable modern CPU hotplug interface: +QEMU starts with modern CPU hotplug interface enabled. Use following steps to +detect modern CPU hotplug interface:
-#. Store 0x0 to the 'CPU selector' register, attempting to switch to modern mode #. Store 0x0 to the 'CPU selector' register, to ensure valid selector value #. Store 0x0 to the 'Command field' register #. Read the 'Command data 2' register. If read value is 0x0, the modern interface is enabled. - Otherwise legacy or no CPU hotplug interface available + Otherwise no CPU hotplug interface available
Get a cpu with pending event ^^^^^^^^^^^^^^^^^^^^^^^^^^^^