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Date: Mon, 04 Mar 2013 12:10:58 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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To: Gleb Natapov <gleb@redhat.com>
CC: Hu Tao <hutao@cn.fujitsu.com>, kvm list <kvm@vger.kernel.org>,
        qemu-devel <qemu-devel@nongnu.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Daniel P. Berrange" <berrange@redhat.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Jan Kiszka <jan.kiszka@siemens.com>, Blue Swirl <blauwirbel@gmail.com>,
        Eric Blake <eblake@redhat.com>, Andrew Jones <drjones@redhat.com>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Sasha Levin <levinsasha928@gmail.com>,
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        Wen Congyang <wency@cn.fujitsu.com>,
        "Michael S. Tsirkin" <mst@redhat.com>, Alexander Graf <agraf@suse.de>,
        Alex Williamson <alex.williamson@redhat.com>,
        Peter Maydell <peter.maydell@linaro.org>
Subject: Re: [PATCH v13 0/8] pv event interface between host and guest
References: <cover.1362051581.git.hutao@cn.fujitsu.com> <20130303091738.GB23616@redhat.com> <513471F1.5020702@redhat.com> <20130304102131.GK23616@redhat.com> <51347735.9090204@redhat.com> <20130304104301.GL23616@redhat.com> <51347C23.8080802@redhat.com> <20130304105928.GM23616@redhat.com>
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Il 04/03/2013 11:59, Gleb Natapov ha scritto:
> > I want to make the ioport configurable in the device, but the PIIX and
> > ICH9 (which are what the DSDT is written for) will always use port 0x505.
> 
> But the device is not part of PIIX or ICH9.

So is kvmclock, or kvmvapic.  I think it makes sense to add this device
to PIIX or ICH9 since it is an ISA device.

> It is additional device that
> may or may not be present depending on a command line. So what if
> someone configures debugcon or debugexit to use this port?

I haven't checked if debug{con,exit} will pass the _STA test, but if
they do, the user will get a Ctrl-A or respectively an exit of QEMU when
the guest panics.

What if someone configures debugcon on port 0x3f8?  Some guest will use
it, some will not.

> We can always
> blame the users, but I fill that we are making unnecessary compromises.

Once we choose an ISA device, where the user has full control of the
address space, we already know we'll have to accept compromises.  I
don't think this compromise is particularly bad: do discovery via ACPI
(nice), accept that the user can trick the AML (ugly).

Paolo

> > You can configure a different iobase for your serial ports, the guest
> > can still use them but not discover them via ACPI.  This is the same thing.
> 
> Probably we should patch DSDT too when it will be in QEMU :) of force
> iobase to spec values if device is used as part of a chipset.