Re: [PATCH 3/6] x86: cpuhp: refuse cpu hot-unplug request earlier if not supported

[Laszlo Ersek <lersek@redhat.com>]

On 07/20/20 16:16, Igor Mammedov wrote:
> CPU hot-unplug with SMM requires firmware participation to prevent
> guest crash (i.e. CPU can be removed only after OS _and_ firmware
> were prepared for th action).

(1) s/th action/the action/

> Previous patches introduced ICH9_LPC_SMI_F_CPU_HOT_UNPLUG_BIT
> feature bit, which is advertised by firmware when it has support
> for CPU hot-unplug. Use it to check if guest is able to handle
> unplug and make device_del fail gracefully if hot-unplug feature
> hasn't been negotiated.
> 
> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> ---
>  hw/acpi/ich9.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/hw/acpi/ich9.c b/hw/acpi/ich9.c
> index 0acc9a3107..98fc363186 100644
> --- a/hw/acpi/ich9.c
> +++ b/hw/acpi/ich9.c
> @@ -460,6 +460,17 @@ void ich9_pm_device_unplug_request_cb(HotplugHandler *hotplug_dev,
>                                        errp);
>      } else if (object_dynamic_cast(OBJECT(dev), TYPE_CPU) &&
>                 !lpc->pm.cpu_hotplug_legacy) {
> +        uint64_t negotiated = lpc->smi_negotiated_features;
> +
> +        if (negotiated & BIT_ULL(ICH9_LPC_SMI_F_BROADCAST_BIT) &&
> +            !(negotiated & BIT_ULL(ICH9_LPC_SMI_F_CPU_HOT_UNPLUG_BIT))) {
> +            error_setg(errp, "cpu hot-unplug with SMI wasn't enabled "
> +                             "by firmware");
> +            error_append_hint(errp, "update machine type to newer than 5.1 "
> +                "and firmware that suppors CPU hot-unplug with SMM");

(2) I think the "machine type to newer than 5.1" reference should be removed at this point. (Similarly to how "x-smi-cpu-hotunplug" should default to "false" at this point in time, in patch#1.) We don't know when hot-unplug will work; we only know that it doesn't work now.

We could say something like

            error_append_hint(errp, "update machine type to a version having "
                              "x-smi-cpu-hotunplug=on and firmware that "
                              "suppors CPU hot-unplug with SMM");


> +            return;
> +        }
> +
>          acpi_cpu_unplug_request_cb(hotplug_dev, &lpc->pm.cpuhp_state,
>                                     dev, errp);
>      } else {
> 

These superficial comments aside, the patch works for me:

Tested-by: Laszlo Ersek <lersek@redhat.com>

and with (1) and (2) fixed:

Reviewed-by: Laszlo Ersek <lersek@redhat.com>

Thanks
Laszlo