Re: [Xen-devel] [RFC] Improving CPU hotplug for 4.3 with qemu-xen

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 19 Jul 2013, Anthony PERARD wrote:
> Hi,
> 
> The cpu hotplug that have been backported from QEMU to qemu-xen (1.5 to
> 1.3) is incomplete. Especially, there is no way to know if a CPU is
> already marked as online by qemu. This result by those error messages
> from libxl:
> 
> libxl: error: libxl_qmp.c:289:qmp_handle_error_response: received an
> error message from QMP server: Unable to add CPU: 0, it already exists
> libxl: error: libxl_qmp.c:289:qmp_handle_error_response: received an
> error message from QMP server: Unable to add CPU: 1, it already exists
> 
> So I tried to backport more patches so it will become possible for libxl
> to get the list of online cpu.
> 
> But this result in *a lot* of patches.
> 
> Here is the git tree and see the list of patches at the end of the mail:
> git://xenbits.xen.org/people/aperard/qemu-dm.git  backport-cpu-device
> 
> There is still some work to be done like making sure everything is
> working properly and making some patches to use it with libxl.
> 
> So George, is it worth getting those patches into Xen 4.3 to get ride of
> the error message ?
> 
> 
> 
> cpu: Change parent type to Device
> qdev: Fold state enum into bool realized
> qdev: Prepare "realized" property
> cpu: Prepare QOM realizefn
> target-i386: Update X86CPU to QOM realizefn
> cpu: Introduce cpu_resume(), for single CPU
> target-i386: Split out CPU creation and features parsing
> cpu: Resume CPU from DeviceClass::realize() if hot-plugged
> Revert "xen: Implement hot_add_cpu hook."
> Revert "acpi_piix4: Add infrastructure to send CPU hot-plug GPE to guest"
> cpu: Introduce CPU hot-plug notifier
> cpu: Add helper cpu_exists(), to check if CPU with specified id exists
> acpi_piix4: Add infrastructure to send CPU hot-plug GPE to guest
> target-i386: Introduce feat2prop() for CPU properties
> target-i386: Introduce apic-id CPU property
> target-i386: Do not allow to set apic-id once CPU is realized
> target-i386: Replace MSI_SPACE_SIZE with APIC_SPACE_SIZE
> qapi/qmp-registry.c: Include headers it needs
> qga/channel-posix.c: Include headers it needs
> user: Move *-user/qemu-types.h to main directory
> user: Rename qemu-types.h to qemu-user-types.h
> Create qemu-types.h for struct typedefs
> qlist.h: Do not include qemu-common.h
> sysbus: make SysBusDeviceClass::init optional
> target-i386: Introduce ICC bus/device/bridge
> target-i386: Attach ICC bus to CPU on its creation
> a fix for cpu_x86_create
> fix machine initialization.

These two commits worry me.
I have a vague recollection that we hit both issues at some point in
the past and we fixed them properly. I would rather have the proper fix
than these two hacks if it's possible.


> extract/unify the constant 0xfee00000 as APIC_DEFAULT_ADDRESS
> kvmvapic: Make dependency on sysbus.h explicit
> target-i386: Move APIC to ICC bus
> pc: Implement QEMUMachine::hot_add_cpu hook

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