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>
---
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
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-- 
2.34.1