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Re: [Qemu-devel] [PATCH 28/35] kvm: x86: Introduce kvmclock device to sa


From: Daniel P. Berrange
Subject: Re: [Qemu-devel] [PATCH 28/35] kvm: x86: Introduce kvmclock device to save/restore its state
Date: Wed, 19 Jan 2011 18:53:50 +0000
User-agent: Mutt/1.5.21 (2010-09-15)

On Wed, Jan 19, 2011 at 11:42:18AM -0600, Anthony Liguori wrote:
> On 01/19/2011 11:35 AM, Daniel P. Berrange wrote:
> >On Wed, Jan 19, 2011 at 10:53:30AM -0600, Anthony Liguori wrote:
> >>On 01/19/2011 03:48 AM, Gerd Hoffmann wrote:
> >>>On 01/18/11 18:09, Anthony Liguori wrote:
> >>>>On 01/18/2011 10:56 AM, Jan Kiszka wrote:
> >>>>>>The device model topology is 100% a hidden architectural detail.
> >>>>>This is true for the sysbus, it is obviously not the case for PCI and
> >>>>>similarly discoverable buses. There we have a guest-explorable topology
> >>>>>that is currently equivalent to the the qdev layout.
> >>>>But we also don't do PCI passthrough so we really haven't even explored
> >>>>how that maps in qdev. I don't know if qemu-kvm has attempted to
> >>>>qdev-ify it.
> >>>It is qdev-ified.  It is a normal pci device from qdev's point of view.
> >>>
> >>>BTW: is there any reason why (vfio-based) pci passthrough couldn't
> >>>work with tcg?
> >>>
> >>>>The -device interface is a stable interface. Right now, you don't
> >>>>specify any type of identifier of the pci bus when you create a PCI
> >>>>device. It's implied in the interface.
> >>>Wrong.  You can specify the bus you want attach the device to via
> >>>bus=<name>.  This is true for *every* device, including all pci
> >>>devices.  If unspecified qdev uses the first bus it finds.
> >>>
> >>>As long as there is a single pci bus only there is simply no need
> >>>to specify it, thats why nobody does that today.
> >>Right.  In terms of specifying bus=, what are we promising re:
> >>compatibility?  Will there always be a pci.0?  If we add some
> >>PCI-to-PCI bridges in order to support more devices, is libvirt
> >>support to parse the hierarchy and figure out which bus to put the
> >>device on?
> >The answer to your questions probably differ depending on
> >whether '-nodefconfig' and '-nodefaults' are set on the
> >command line.  If they are set, then I'd expect to only
> >ever see one PCI bus with name pci.0 forever more, unless
> >i explicitly ask for more. If they are not set, then you
> >might expect to see multiple PCI buses by appear by magic
> 
> Yeah, we can't promise that.  If you use -M pc, you aren't
> guaranteed a stable PCI bus topology even with
> -nodefconfig/-nodefaults.

That's why we never use '-M pc' when actually invoking QEMU.
If the user specifies 'pc' in the XML, we canonicalize that
to the versioned alternative like 'pc-0.12' before invoking
QEMU. We also expose the list of versioned machines to apps
so they can do canonicalization themselves if desired.

Regards,
Daniel



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