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+QEMU<->ACPI BIOS memory hotplug interface
+--------------------------------------
+
+ACPI BIOS GPE.3 handler is dedicated for notifying OS about memory hot-add
+and hot-remove events.
+
+Memory hot-plug interface (IO port 0xa00-0xa17, 1-4 byte access):
+---------------------------------------------------------------
+0xa00:
+ read access:
+ [0x0-0x3] Lo part of memory device phys address
+ [0x4-0x7] Hi part of memory device phys address
+ [0x8-0xb] Lo part of memory device size in bytes
+ [0xc-0xf] Hi part of memory device size in bytes
+ [0x10-0x13] Memory device proximity domain
+ [0x14] Memory device status fields
+ 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 1 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
+ [0x15-0x17] reserved
+
+ write access:
+ [0x0-0x3] Memory device slot selector, selects active memory device.
+ All following accesses to other registers in 0xa00-0xa17
+ region will read/store data from/to selected memory device.
+ [0x4-0x7] OST event code reported by OSPM
+ [0x8-0xb] OST status code reported by OSPM
+ [0xc-0x13] reserved, writes into it are ignored
+ [0x14] Memory device control fields
+ bits:
+ 0: reserved, OSPM must clear it before writing to register.
+ Due to BUG in versions prior 2.4 that field isn't cleared
+ when other fields are written. Keep it reserved and don't
+ try to reuse it.
+ 1: if set to 1 clears device insert event, set by OSPM
+ after it has emitted device check event for the
+ selected memory device
+ 2: if set to 1 clears device remove event, set by OSPM
+ after it has emitted device eject request for the
+ selected memory device
+ 3: if set to 1 initiates device eject, set by OSPM when it
+ triggers memory device removal and calls _EJ0 method
+ 4-7: reserved, OSPM must clear them before writing to register
+
+Selecting memory device slot beyond present range has no effect on platform:
+ - write accesses to memory hot-plug registers not documented above are
+ ignored
+ - read accesses to memory hot-plug registers not documented above return
+ all bits set to 1.
+
+Memory hot remove process diagram:
+----------------------------------
+ +-------------+     +-----------------------+      +------------------+     
+ |  1. QEMU    |     | 2. QEMU               |      |3. QEMU           |     
+ |  device_del +---->+ device unplug request +----->+Send SCI to guest,|     
+ |             |     |         cb            |      |return control to |     
+ +-------------+     +-----------------------+      |management        |     
+                                                    +------------------+     
+                                                                             
+ +---------------------------------------------------------------------+     
+                                                                             
+ +---------------------+              +-------------------------+            
+ | OSPM:               | remove event | OSPM:                   |            
+ | send Eject Request, |              | Scan memory devices     |            
+ | clear remove event  +<-------------+ for event flags         |            
+ |                     |              |                         |            
+ +---------------------+              +-------------------------+            
+           |                                                                 
+           |                                                                 
+ +---------v--------+            +-----------------------+                   
+ | Guest OS:        |  success   | OSPM:                 |                   
+ | process Ejection +----------->+ Execute _EJ0 method,  |                   
+ | request          |            | set eject bit in flags|                   
+ +------------------+            +-----------------------+                   
+           |failure                         |                                
+           v                                v                                
+ +------------------------+      +-----------------------+                   
+ | OSPM:                  |      | QEMU:                 |                   
+ | set OST event & status |      | call device unplug cb |                   
+ | fields                 |      |                       |                   
+ +------------------------+      +-----------------------+                   
+          |                                  |                               
+          v                                  v                               
+ +------------------+              +-------------------+                     
+ |QEMU:             |              |QEMU:              |                     
+ |Send OST QMP event|              |Send device deleted|                     
+ |                  |              |QMP event          |                     
+ +------------------+              |                   |                     
+                                   +-------------------+