Xen devel - Mar 2013 - Regression - Xen ACPI CPU hotplug processor causes the Xen ACPI P and C-states to stop working.

The reason is that the Xen CPU hotplug (xen-acpi-cpuhotplug.c)
registers itself as the "processor" type object. That means the
generic processor (processor_driver.c) stops working and it does
not call (acpi_processor_add) which populates the
         per_cpu(processors, pr->id) = pr;

structure. Previous to the population (acpi_processor_add) also calls
''acpi_processor_get_info'' which does the job of finding the
C-states and
figuring out PBLK address and all of that ends up in the ''pr''
structure.

The ''processors->pr'' is then used by Xen ACPI Processor
(the one that
uploads C and P states to the hypervisor) to gather the data. Since it
is NULL, it skips the gathering of _PSD, _PSS, _PCT, etc and we never
end up uploading said data.

The end is that enabling the CONFIG_XEN_STUB means that xen-acpi-processor
is not working anymore.

This temporary patch:


diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
index 5a32232..67af155 100644
--- a/drivers/xen/Kconfig
+++ b/drivers/xen/Kconfig
@@ -182,7 +182,7 @@ config XEN_PRIVCMD
 
 config XEN_STUB
 	bool "Xen stub drivers"
-	depends on XEN && X86_64
+	depends on XEN && X86_64 && BROKEN
 	default n
 	help
 	  Allow kernel to install stub drivers, to reserve space for Xen drivers,

fixes the regression, but this problem needs to be fixed long-term.