Re: [Question] x86/microvm: why has_hotpluggable_cpus = false but hot(ub)plug APIs exist?

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 18 Oct 2023 17:48:36 +0100
Salil Mehta <salil.mehta@opnsrc.net> wrote:

> Hi Alex,
> 
> On 18/10/2023 16:41, Alex Bennée wrote:
> > 
> > Salil Mehta <salil.mehta@opnsrc.net> writes:
> >   
> >> Hello,
> >>
> >> Came across below code excerpt in x86/microvm code and wanted to know
> >> why 'has_hotpluggable_cpus' flag has been set to 'false' while various
> >> hot(un)plug APIs have been defined?
> >>
> >> static void microvm_class_init(ObjectClass *oc, void *data)
> >> {
> >>      X86MachineClass *x86mc = X86_MACHINE_CLASS(oc);
> >>      MachineClass *mc = MACHINE_CLASS(oc);
> >>      HotplugHandlerClass *hc = HOTPLUG_HANDLER_CLASS(oc);
> >>
> >>      mc->init = microvm_machine_state_init;
> >>
> >>      mc->family = "microvm_i386";
> >>      [...]
> >>      mc->max_cpus = 288;
> >>      mc->has_hotpluggable_cpus = false;  --------> This one
> >>      [...]  
> > 
> >  From the original commit that added it:
> > 
> >    It's a minimalist machine type without PCI nor ACPI support, designed
> >    for short-lived guests. microvm also establishes a baseline for
> >    benchmarking and optimizing both QEMU and guest operating systems,
> >    since it is optimized for both boot time and footprint.  
> 
> 
> Agreed. It looks like ACPI is supported but neither CPU/Memory Hotplug 
> is supported for this minimalist machine type.
> 
> 
> static void microvm_devices_init(MicrovmMachineState *mms)
> {
>      const char *default_firmware;
>      X86MachineState *x86ms = X86_MACHINE(mms);
> 
>     [...]
> 
>      /* Optional and legacy devices */
>      if (x86_machine_is_acpi_enabled(x86ms)) {
>          DeviceState *dev = qdev_new(TYPE_ACPI_GED_X86);
>          qdev_prop_set_uint32(dev, "ged-event", ACPI_GED_PWR_DOWN_EVT);
>          sysbus_mmio_map(SYS_BUS_DEVICE(dev), 0, GED_MMIO_BASE);
>       /* sysbus_mmio_map(SYS_BUS_DEVICE(dev), 1, GED_MMIO_BASE_MEMHP); */
> 
>          [...]
> 
>          sysbus_realize(SYS_BUS_DEVICE(dev), &error_fatal);
>          x86ms->acpi_dev = HOTPLUG_HANDLER(dev);
>      }
>     [...]
> }
> 
> 
> 
> > 
> > Generally hotplug requires a dance between the VMM and the firmware to
> > properly shutdown and restart hotplug devices. The principle
> > communication mechanism for this is ACPI.  

firmware part in cpu/mem hoptlug usually provided by QEMU by the way of
ACPI tables (which may contain AML code that handles dance with QEMU
while exposing standard interface to guest OS to handle hotplug)

> 
> Agreed.
> 
> 
> 
> >   
> >>
> >>      /* hotplug (for cpu coldplug) */
> >>      mc->get_hotplug_handler = microvm_get_hotplug_handler;
> >>      hc->pre_plug = microvm_device_pre_plug_cb;
> >>      hc->plug = microvm_device_plug_cb;
> >>      hc->unplug_request = microvm_device_unplug_request_cb;
> >>      hc->unplug = microvm_device_unplug_cb;  
> 
> sorry, I also missed the definitions of the last 2 functions which says 
> that unplug is not supported so perhaps these functions are only 
> required to support cold plugging which corroborates with the comment as 
> well.

this function are usually used for both cold and hotplug of bus-less devices.
They provide an opt-in way for board to wire up such devices (incl. CPU).

> 
> Thanks
> Salil.
>