docs/specs/acpi_cpu_hotplug.rst

1 QEMU<->ACPI BIOS CPU hotplug interface
4 QEMU supports CPU hotplug via ACPI. This document
7 ACPI BIOS GPE.2 handler is dedicated for notifying OS about CPU hot-add
11 Legacy ACPI CPU hotplug interface registers
14 CPU present bitmap for:
18 - One bit per CPU. Bit position reflects corresponding CPU APIC ID. Read-only.
20   to modern CPU hotplug interface, write 0 into it to do switch.
22 QEMU sets corresponding CPU bit on hot-add event and issues SCI
23 with GPE.2 event set. CPU present map is read by ACPI BIOS GPE.2 handler
24 to notify OS about CPU hot-add events. CPU hot-remove isn't supported.
27 Modern ACPI CPU hotplug interface registers
46 The last stored value in 'CPU selector' must refer to a possible CPU, otherwise
51 On QEMU start, 'CPU selector' is initialized to a valid value, on reset it
65     upper 32 bits of architecture specific CPU ID value
70   CPU device status fields: (1 byte access)
100     contains 'CPU selector' value of a CPU with pending event[s]
102     lower 32 bits of architecture specific CPU ID value
111   CPU selector: (DWORD access)
113   Selects active CPU device. All following accesses to other
114   registers will read/store data from/to selected CPU.
118   CPU device control fields: (1 byte access)
127     selected CPU device
131     selected CPU device.
134     triggers CPU device removal and calls _EJ0 method or by firmware
141     it's asked firmware to perform CPU device eject.
151     selects a CPU device with inserting/removing events and
153     selected CPU ('CPU selector' value).
154     If no CPU with events found, the current 'CPU selector' doesn't
165     architecture specific CPU ID value for currently selected CPU.
189 (x86) Detecting and enabling modern CPU hotplug interface
192 QEMU starts with legacy CPU hotplug interface enabled. Detecting and
193 switching to modern interface is based on the 2 legacy CPU hotplug features:
195 #. Writes into CPU bitmap are ignored.
196 #. CPU bitmap always has bit #0 set, corresponding to boot CPU.
198 Use following steps to detect and enable modern CPU hotplug interface:
200 #. Store 0x0 to the 'CPU selector' register, attempting to switch to modern mode
201 #. Store 0x0 to the 'CPU selector' register, to ensure valid selector value
205    Otherwise legacy or no CPU hotplug interface available
210 #. Store 0x0 to the 'CPU selector' register.
212 #. Read the 'CPU device status fields' register.
213 #. If both bit #1 and bit #2 are clear in the value read, there is no CPU
214    with a pending event and selected CPU remains unchanged.
216    selector of the CPU with the pending event (which is already selected).
221 #. Set the present CPU count to 0.
223 #. Store 0x0 to the 'CPU selector' register, to ensure that it's in
226    register return 'CPU selector' value of selected CPU
227 #. Read the 'CPU device status fields' register.
228 #. If bit #0 is set, increment the present CPU count.
230 #. Store the iterator to the 'CPU selector' register.
233 #. Otherwise store 0x0 to the 'CPU selector' register, to put it