docs: update ACPI CPU hotplug spec with new protocol

Add description of new CPU hotplug interface.

To switch from from legacy mode into new mode use fact
that write accesses into CPU present bitmap were never
used before and were ignored by QEMU.
So use it to as a way to switch from legacy mode.
That way pc/q35 machine starts in legacy mode and
QEMU generated ACPI tables will switch to new CPU
hotplug interface during runtime.
In case QEMU is started with legacy BIOS (that doesn't
support QEMU generated ACPI tables), legacy CPU hotplug
will remain active and could be used by BIOS built in
ACPI tables for CPU hotplug.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

docs/specs/acpi_cpu_hotplug.txt

@@ -4,21 +4,91 @@ 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:
   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 new CPU hotplug interface, write 0 into it to do switch.
 ---------------------------------------------------------------
-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 is 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: selects a CPU device with inserting/removing events and
+               following reads from 'Command data' register return
+               selected CPU (CPU selector value). If no CPU with events
+               found, the current CPU selector doesn't change and
+               corresponding insert/remove event flags are not set.
+            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:
+              0: OSPM reads value of CPU selector
+              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 0.