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Date: Wed, 6 Mar 2013 11:38:53 +0200
From: Gleb Natapov <gleb@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v13 0/8] pv event interface between host
	and guest
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Peter Maydell <peter.maydell@linaro.org>, Andrew Jones <drjones@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, Alex Williamson <alex.williamson@redhat.com>, kvm list <kvm@vger.kernel.org>, "Michael S. Tsirkin" <mst@redhat.com>, Hu Tao <hutao@cn.fujitsu.com>, Juan Quintela <quintela@redhat.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, qemu-devel <qemu-devel@nongnu.org>, Markus Armbruster <armbru@redhat.com>, Blue Swirl <blauwirbel@gmail.com>, Orit Wasserman <owasserm@redhat.com>, Alexander Graf <agraf@suse.de>, Sasha Levin <levinsasha928@gmail.com>, Stefan Hajnoczi <stefanha@redhat.com>, Jan Kiszka <jan.kiszka@siemens.com>, Luiz Capitulino <lcapitulino@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

On Wed, Mar 06, 2013 at 10:07:31AM +0100, Paolo Bonzini wrote:
> Il 06/03/2013 09:56, Hu Tao ha scritto:
> >> > 
> >> > Something like this should work (in SeaBIOS's src/acpi-dsdt-isa.dsl):
> >> > 
> >> >     Device(PEVT) {
> >> >         Name(_HID, EisaId("QEMU0001"))
> >> >         OperationRegion(PEOR, SystemIO, 0x505, 0x01)
> >> >         Field(PEOR, ByteAcc, NoLock, Preserve) {
> >> >             PEPT,   8,
> >> >         }
> >> > 
> >> >         Method(_STA, 0, NotSerialized) {
> >> >             Store(PEPT, Local0)
> >> >             If (LEqual(Local0, Zero)) {
> >> >                 Return (0x00)
> >> >             } Else {
> >> >                 Return (0x0F)
> >> >             }
> >> >         }
> > IIUC, here _STA reads from ioport 0x505, if the result is 0, then the
> > device is not present. Otherwise, the device is present. But as Gleb
> > said, ''the data you read from unassigned port is not guarantied to be
> > zero, it may depend on QEMU version''. What should I do to tell if the
> > device is present or not correctly?
> 
> The firmware is tied to the QEMU version, so you can rely on unassigned
> ports returning zero.
> 
> Later we can change this to use fw-cfg.
> 
I thought we agreed to do it from the start :)

--
			Gleb.