On Wed, 3 Dec 2025 00:28:13 +0800 Zhao Liu <[email protected]> wrote:
> Legacy cpu hotplug has been removed totally and machines start with > modern cpu hotplug interface directly. > > Therefore, update the documentation to describe current QEMU cpu hotplug > logic. > > Signed-off-by: Zhao Liu <[email protected]> Reviewed-by: Igor Mammedov <[email protected]> > --- > Changes since v4: > * New patch. > --- > docs/specs/acpi_cpu_hotplug.rst | 28 +++------------------------- > 1 file changed, 3 insertions(+), 25 deletions(-) > > diff --git a/docs/specs/acpi_cpu_hotplug.rst b/docs/specs/acpi_cpu_hotplug.rst > index 351057c96761..f49678100044 100644 > --- a/docs/specs/acpi_cpu_hotplug.rst > +++ b/docs/specs/acpi_cpu_hotplug.rst > @@ -8,22 +8,6 @@ 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 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. > - > - > Modern ACPI CPU hotplug interface registers > ------------------------------------------- > > @@ -189,20 +173,14 @@ 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: > - > -#. Writes into CPU bitmap are ignored. > -#. CPU bitmap always has bit #0 set, corresponding to boot CPU. > - > -Use following steps to detect and enable modern CPU hotplug interface: > +QEMU starts with modern CPU hotplug interface enabled. Use following steps to > +detect modern CPU hotplug interface: > > -#. Store 0x0 to the 'CPU selector' register, attempting to switch to modern > mode > #. Store 0x0 to the 'CPU selector' register, to ensure valid selector value > #. Store 0x0 to the 'Command field' register > #. Read the 'Command data 2' register. > If read value is 0x0, the modern interface is enabled. > - Otherwise legacy or no CPU hotplug interface available > + Otherwise no CPU hotplug interface available > > Get a cpu with pending event > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
