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Date: Tue, 11 Jun 2013 09:42:29 +0200
From: Gerd Hoffmann <kraxel@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] qemu: piix: PCI bridge ACPI hotplug support
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, Anthony Liguori <anthony@codemonkey.ws>, David Woodhouse <dwmw2@infradead.org>, seabios@seabios.org, qemu-devel@nongnu.org, Kevin O'Connor <kevin@koconnor.net>, Jordan Justen <jljusten@gmail.com>, Laszlo Ersek <lersek@redhat.com>

  Hi,

>>> Portability:
>>>     - Non x86 (or any Linux) platforms don't need any of this code.
>>>       They can keep happily using SHPC the way
>>>       they always did.
>>
>> Hmm.  Is is possible to write a SHPC driver in AML?  I think it would be
>> alot better to have one guest/host interface for pci bridge hotplug
>> instead of two.
>>
>> cheers,
>>   Gerd
> 
> No, it's not possible, SHPC is not designed to be used from ACPI.
> 
> Two reasons off the top of my head, there are likely others:
> 
> 1. SHPC uses regular PCI interrupts to signal events. It does not signal
>    GFE and SCI.
> 
> 2. SHPC uses config accesses to get information from device.
>    ACPI does not allow config access anywhere except the root bus from ACPI
>    (This requirement is designed to give the OS freedom
>    to reconfigure PCI in an arbitrary way).

OK, so it's designed for OSes to have native SHPC support.  Linux has that?

Quick googling found me Windows Vista+ has it too, correct?  So that
leaves Win2k + WinXP versions.  Older Windows versions do not support
pci hotplug at all.  Win2k is EOL already.  WinXP will follow soon.

More users?

/me wonders whenever it is worth hopping through the loops needed to
support ACPI-based hotplug of devices behind bridges in the first place.

cheers,
  Gerd