piix4acpi, xen, hotplug: Fix race with ACPI AML code and hotplug.

piix4acpi, xen, hotplug: Fix race with ACPI AML code and hotplug.

This is a race so the amount varies but on a 4PCPU box
I seem to get only ~14 out of 16 vCPUs I want to online.

The issue at hand is that QEMU xenstore.c hotplug code changes
the vCPU array and triggers an ACPI SCI for each vCPU
online/offline change. That means we modify the array of vCPUs
as the guests ACPI AML code is reading it - resulting in
the guest reading the data only once and not changing the
CPU states appropiately.

The fix is to seperate the vCPU array changes from the ACPI SCI
notification. The code now will enumerate all of the vCPUs
and change the vCPU array if there is a need for a change.
If a change did occur then only _one_ ACPI SCI pulse is sent
to the guest. The vCPU array at that point has the online/offline
modified to what the user wanted to have.

Specifically, if a user provided this command:
 xl vcpu-set latest 16

(guest config has vcpus=1, maxvcpus=32) QEMU and the guest
(in this case Linux) would do:

QEMU:                                           Guest OS:
-xenstore_process_vcpu_set_event
 -> Gets an XenBus notification for CPU1
 -> Updates the gpe_state.cpus_state bitfield.
        -> Pulses the ACPI SCI
                                                - ACPI SCI kicks in

 -> Gets an XenBus notification for CPU2
 -> Updates the gpe_state.cpus_state bitfield.
        -> Pulses the ACPI SCI

 -> Gets an XenBus notification for CPU3
 -> Updates the gpe_state.cpus_state bitfield.
        -> Pulses the ACPI SCI
   ...
                                                 - Method(PRST) invoked

 -> Gets an XenBus notification for CPU12
 -> Updates the gpe_state.cpus_state bitfield.
        -> Pulses the ACPI SCI
                                                  - reads AF00 for CPU state
                                                    [gets 0xff]
                                                  - reads AF02 [gets 0x7f]

 -> Gets an XenBus notification for CPU13
 -> Updates the gpe_state.cpus_state bitfield.
        -> Pulses the ACPI SCI

        .. until VCPU 16
                                                 - Method PRST updates
                                                   PR01 through 13 FLG
                                                   entry.
                                                 - PR01->PR13 _MAD
                                                   invoked.

                                                 - Brings up 13 CPUs.

While QEMU updates the rest of the cpus_state bitfields the ACPI AML
only does the CPU hotplug on those it had read.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com> (for 4.3 release)
(cherry picked from commit f62079cd7de6ec37f48dfc80fb5906f49fecd6f6)