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Subject: Re: Configuration vs. compat hints [was Re: [Qemu-devel] [PATCHv3
	03/13] qemu: add routines to manage PCI capabilities]
From: Mark McLoughlin <markmc@redhat.com>
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Date: Mon, 15 Jun 2009 10:08:36 +0100
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Reply-To: Mark McLoughlin <markmc@redhat.com>
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Carsten Otte <cotte@de.ibm.com>, kvm@vger.kernel.org, Glauber Costa <glommer@redhat.com>, Rusty Russell <rusty@rustcorp.com.au>, qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org, Blue Swirl <blauwirbel@gmail.com>, Christian Borntraeger <borntraeger@de.ibm.com>, Paul Brook <paul@codesourcery.com>, Avi Kivity <avi@redhat.com>

On Sun, 2009-06-14 at 12:50 +0300, Michael S. Tsirkin wrote:
> On Fri, Jun 12, 2009 at 05:48:23PM +0100, Mark McLoughlin wrote:
> > However, in order to retain compat for that SCSI device (e.g. ensuring
> > the PCI address doesn't change as other devices are added an removed),
> > we're back to the same problem ... either:
> > 
> >   1) Use '-drive file=foo.img,if=scsi,pci_addr=foo'; in order to figure 
> >      out what address to use, libvirt would need to query qemu for what 
> >      address was originally allocated to device or it would do all the 
> >      PCI address allocation itself ... 
> 
> This last option makes sense to me: in a real world the user has
> control over where he places the device on the bus, so why
> not with qemu?

Yep, most people seem to agree that it makes sense to allow this, but
some believe it should only be via a machine description file, not the
command line.

However, the first problem is that it isn't a solution to the guest ABI
problem more generally.

And the second problem is that for e.g. libvirt to use it, it would have
to be possible to query qemu for what PCI slots were assigned to the
devices - libvirt would need to be able to parse 'info pci' and match
the devices listed with the devices specified on the command line.

Again, details written up here:

  https://fedoraproject.org/wiki/Features/KVM_Stable_PCI_Addresses

Cheers,
Mark.