Describe how to enable and detect modern CPU hotplug interface.
Detection part is based on new CPHP_GET_CPU_ID_CMD command,
introduced by "acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command" patch.
Signed-off-by: Igor Mammedov <imamm...@redhat.com>
---
docs/specs/acpi_cpu_hotplug.txt | 22 ++++++++++++++++++++--
1 file changed, 20 insertions(+), 2 deletions(-)
diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt
index bb33144..667b264 100644
--- a/docs/specs/acpi_cpu_hotplug.txt
+++ b/docs/specs/acpi_cpu_hotplug.txt
@@ -15,14 +15,14 @@ CPU present bitmap for:
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.
+ 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.
=====================================
-ACPI CPU hotplug interface registers:
+Modern ACPI CPU hotplug interface registers:
-------------------------------------
Register block base address:
ICH9-LPC IO port 0x0cd8
@@ -105,6 +105,24 @@ write access:
other values: reserved
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:
+ 1. Writes into CPU bitmap are ignored.
+ 2. CPU bitmap always has bit#0 set, corresponding to boot CPU.
+
+ Use following steps to detect and enable modern CPU hotplug
interface:
+ 1. Store 0x0 to the 'CPU selector' register,
+ attempting to switch to modern mode
+ 2. Store 0x0 to the 'CPU selector' register,
+ to ensure valid selector value
+ 3. Store 0x3 to the 'Command field' register,
+ sets the 'Command data 2' register into architecture specific
+ CPU identifier mode
+ 4. Read the 'Command data 2' register.
+ If read value is 0x0, the modern interface is enabled.
+ Otherwise legacy or no CPU hotplug interface available
+
- Get a cpu with pending event
1. Store 0x0 to the 'CPU selector' register.
2. Store 0x0 to the 'Command field' register.
--
2.7.4
