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Re: [libvirt] [PATCH 2/2] qemu: Update next usable PCI slot on domain reconnect



On Wed, Aug 18, 2010 at 01:15:55PM +0200, Jiri Denemark wrote:
> > Why do we need it to be exactly the same value ?  nextslot is just an
> > efficiency optimization isn't it. ie, so instead of starting from
> > slot 0 and iterating over 'N' already used slots till we find a free
> > slot, we can get the next free slot in 1 step. As such do we really
> > need to worry about restoring it to the same value after restarting
> > libvirtd.
> 
> That was my understanding too. But Eric was concerned (in an older thread)
> about hotplugging PCI devices in a nonmonotonic way. He thinks it could upset
> Windows guests. Of course, if nextslot ever wraps from
> QEMU_PCI_ADDRESS_LAST_SLOT back to zero, such guests would be doomed anyway so
> we are only a bit nicer to them. I don't know if this is a real issue or not
> since I haven't met a Windows guest which I'd like to hotplug a PCI device in.

There's no requirement to plug devices in ascending slot order - we can
have gaps at will with any ordering.

Daniel
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