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Date: Tue, 11 Jun 2013 11:18:13 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] qemu: piix: PCI bridge ACPI hotplug support
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To: Gerd Hoffmann <kraxel@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>

On Tue, Jun 11, 2013 at 10:00:36AM +0200, Gerd Hoffmann wrote:
>   Hi,
> 
> > Yes and not just because of windows guests.
> > ACPI spec is also very explicit that native hotplug is an optional
> > feature. Test suites such as WHQL are known to test spec compliance.
> 
> /me looks a bit surprised.
> 
> This pretty much implies that any shpc bridge needs a second interface
> to the hotplug functionality which can be driven via ACPI.  Or the
> firmware somehow handles this using smm (not sure this works for the IRQ
> though).
> 
> Do you know how this is handled by real hardware?
> 
> cheers,
>   Gerd

SHPC is not very widely deployed on a PC.

Since most hardware vendors do care about windows support,
the only way is a separate interface that is driven via ACPI.
You then need an ACPI specific register to switch to standard SHPC.
The SHPC spec even tells you as much.

-- 
MST