diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt
index 340b751..c5bce6a 100644
--- a/docs/specs/acpi_cpu_hotplug.txt
+++ b/docs/specs/acpi_cpu_hotplug.txt
@@ -4,21 +4,85 @@ QEMU<->ACPI BIOS CPU hotplug interface
 QEMU supports CPU hotplug via ACPI. This document
 describes the interface between QEMU and the ACPI BIOS.
 
-ACPI GPE block (IO ports 0xafe0-0xafe3, byte access):
------------------------------------------
-
-Generic ACPI GPE block. Bit 2 (GPE.2) used to notify CPU
-hot-add/remove event to ACPI BIOS, via SCI interrupt.
+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:
+  One bit per CPU. Bit position reflects corresponding CPU APIC ID. Read-only.
   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.
+QEMU sets corresponding CPU bit on hot-add event and issues SCI
+with GPE.2 event set. CPU present map read by ACPI BIOS GPE.2 handler
+to notify OS about CPU hot-add events. CPU hot-remove isn't supported.
+
+=====================================
+ACPI CPU hotplug interface registers:
+-------------------------------------
+Register block base address:
+    ICH9-LPC IO port 0x0cd8
+    PIIX-PM  IO port 0xaf00
+Register block size:
+    ACPI_CPU_HOTPLUG_REG_LEN = 12
+
+read access:
+    offset:
+    [0x0-0x3] reserved
+    [0x4] CPU device status fields: (1 byte access)
+        bits:
+           0: Device is enabled and may be used by guest
+           1: Device insert event, used to distinguish device for which
+              no device check event to OSPM was issued.
+              It's valid only when bit 0 is set.
+           2: Device remove event, used to distinguish device for which
+              no device eject request to OSPM was issued.
+           3-7: reserved and should be ignored by OSPM
+    [0x5-0x7] reserved
+    [0x8] Command data: (DWORD access)
+          in case of error or unsupported command reads is 0xFFFFFFFF
+          current 'Command field' value:
+              0: returns PXM value corresponding to device
+
+write access:
+    offset:
+    [0x0-0x3] CPU selector: (DWORD access)
+              selects active CPU device. All following accesses to other
+              registers will read/store data from/to selected CPU.
+    [0x4] CPU device control fields: (1 byte access)
+        bits:
+            0: reserved, OSPM must clear it before writing to register.
+            1: if set to 1 clears device insert event, set by OSPM
+               after it has emitted device check event for the
+               selected CPU device
+            2: if set to 1 clears device remove event, set by OSPM
+               after it has emitted device eject request for the
+               selected CPU device
+            3: if set to 1 initiates device eject, set by OSPM when it
+               triggers CPU device removal and calls _EJ0 method
+            4-7: reserved, OSPM must clear them before writing to register
+    [0x5] Command field: (1 byte access)
+          value:
+            0: following reads from 'Command data' register returns PXM
+               value of device
+            1: following writes to 'Command data' register set OST event
+               register in QEMU
+            2: following writes to 'Command data' register set OST status
+               register in QEMU
+            other values: reserved
+    [0x6-0x7] reserved
+    [0x8] Command data: (DWORD access)
+          current 'Command field' value:
+              1: stores value into OST event register
+              2: stores value into OST status register, triggers
+                 ACPI_DEVICE_OST QMP event from QEMU to external applications
+                 with current values of OST event and status registers.
+            other values: reserved
 
-CPU hot-add/remove notification:
------------------------------------------------------
-QEMU sets/clears corresponding CPU bit on hot-add/remove event.
-CPU present map read by ACPI BIOS GPE.2 handler to notify OS of CPU
-hot-(un)plug events.
+Selecting CPU device beyond possible range has no effect on platform:
+   - write accesses to CPU hot-plug registers not documented above are
+     ignored
+   - read accesses to CPU hot-plug registers not documented above return
+     all bits set to 1.
