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


From: Paolo Bonzini
Subject: Re: [PATCH-for-5.0 1/2] hw/acpi/piix4: Add 'system-hotplug-support' property
Date: Thu, 19 Mar 2020 11:02:25 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

On 19/03/20 10:42, Philippe Mathieu-Daudé wrote:
> 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):

Right, I should have been more verbose.  You mentioned I/O port 0xaf00
which is CPU hotplug.  Perhaps unless you can also crash with PCI
hotplug (0xae00-0xae0f) it's worth removing only CPU hotplug from MIPS
machines, and keep PCI hotplug.

Paolo

> 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->use_acpi_pci_hotplug);
> 
>     s->cpu_hotplug_legacy = true;
>     object_property_add_bool(OBJECT(s), "cpu-hotplug-legacy",
>                              piix4_get_cpu_hotplug_legacy,
>                              piix4_set_cpu_hotplug_legacy,
>                              NULL);
>     legacy_acpi_cpu_hotplug_init(parent, OBJECT(s), &s->gpe_cpu,
>                                  PIIX4_CPU_HOTPLUG_IO_BASE);
> 
>     if (s->acpi_memory_hotplug.is_enabled) {
>         acpi_memory_hotplug_init(parent, OBJECT(s),
> &s->acpi_memory_hotplug,
>                                  ACPI_MEMORY_HOTPLUG_BASE);
>     }
> }
> 




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