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Re: [PATCH-for-5.0 1/2] hw/acpi/piix4: Add 'system-hotplug-support' prop

From: Philippe Mathieu-Daudé
Subject: Re: [PATCH-for-5.0 1/2] hw/acpi/piix4: Add 'system-hotplug-support' property
Date: Thu, 19 Mar 2020 10:42:54 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

On 3/19/20 10:36 AM, Paolo Bonzini wrote:
On 18/03/20 23:15, Philippe Mathieu-Daudé wrote:
The I/O ranges registered by the piix4_acpi_system_hot_add_init()
function are not documented in the PIIX4 datasheet.
This appears to be a PC-only feature added in commit 5e3cb5347e
("initialize hot add system / acpi gpe") which was then moved
to the PIIX4 device model in commit 9d5e77a22f ("make
qemu_system_device_hot_add piix independent")
Add a property (default enabled, to not modify the current
behavior) to allow machines wanting to model a simple PIIX4
to disable this feature.

Yes, all hotplug stuff (PCI/memory/CPU) are custom additions by QEMU.

+    DEFINE_PROP_BOOL("system-hotplug-support", PIIX4PMState,
+                     use_acpi_system_hotplug, true),

Why not cpu-hotplug-support?

Because I have no idea what this code is about, and it seems more than cpu (pci, memory):

static void piix4_acpi_system_hot_add_init(MemoryRegion *parent,
                                           PCIBus *bus, PIIX4PMState *s)
    memory_region_init_io(&s->io_gpe, OBJECT(s), &piix4_gpe_ops, s,
                          "acpi-gpe0", GPE_LEN);
    memory_region_add_subregion(parent, GPE_BASE, &s->io_gpe);

    acpi_pcihp_init(OBJECT(s), &s->acpi_pci_hotplug, bus, parent,

    s->cpu_hotplug_legacy = true;
    object_property_add_bool(OBJECT(s), "cpu-hotplug-legacy",
    legacy_acpi_cpu_hotplug_init(parent, OBJECT(s), &s->gpe_cpu,

    if (s->acpi_memory_hotplug.is_enabled) {
acpi_memory_hotplug_init(parent, OBJECT(s), &s->acpi_memory_hotplug,

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