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Date: Mon, 17 Feb 2014 11:38:34 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] Fwd: Trying to write a new device / virtio-i2c ?
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To: Alex David <alex.daerf@gmail.com>, qemu-devel <qemu-devel@nongnu.org>

Il 17/02/2014 11:01, Alex David ha scritto:
> I indeed don't use paravirtualization.

Virtio _is_ paravirtualization. :)

> I'm emulating a bunch of sensors/actuators.
>
> If I virtualize my sensors and attach them to the i2c-dev with -device,
> how do I get those data on the host then ?

It depends on your use case.

It could be that you can make them return a constant value.

Otherwise, you may want to use a chardev for that purpose, or finally a 
QOM (QEMU Object Model) property.  For example, add

CONFIG_TMP105=y

to default-configs/x86_64-softmmu.mak before building QEMU, then do the 
following (indented = in the guest):

$ x86_64-softmmu/qemu-system-x86_64 --enable-kvm ~/test2.img  -m 256 \
    -device tmp105,id=sensor,address=0x50 \
    -qmp unix:$HOME/qmp.sock,server,nowait
$ qmp/qom-list -s ~/qmp.sock /machine/peripheral/sensor
temperature
@parent_bus/
address
hotpluggable
realized
type
$ scripts/qmp/qmp-shell ~/qmp.sock
(QEMU) qom-get path=/machine/peripheral/sensor property=temperature
{u'return': 0}
(QEMU) qom-get path=sensor property=address
{u'return': 80}

     # modprobe i2c-dev
     # i2cget -y 0 0x50 0 w
     0x0000

(QEMU) qom-set path=sensor property=temperature value=20000
{u'return': {}}

     # i2cget -y 0 0x50 0 w
     0x0014

For this particular sensor, you have to swap the two bytes and the 
result is 8.8 fixed-point.

Paolo

> Thanks for your help. As you may see, I'm not that experienced in
> QEMU/Linux kernel.