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Re: [Qemu-devel] [PATCH] Change virtio-console to PCI_CLASS_SERIAL_OTHER

From: Dor Laor
Subject: Re: [Qemu-devel] [PATCH] Change virtio-console to PCI_CLASS_SERIAL_OTHER
Date: Fri, 29 May 2009 00:45:02 +0300
User-agent: Thunderbird (X11/20090105)

Anthony Liguori wrote:
Paul Brook wrote:
  - the device model cannot change or the guest OS will get confused
IMHO think the only sane response is "don't do that". Trying to support
migration between different qemu versions just isn't worth the pain.
It is very worth the pain.  I consider it a core requirement.
We disagree then. You're effectively requiring bug-compatibility.

You can take it to an extreme and require bug-compatibility, but I don't
think that's necessary.  I think there's room for being reasonable.

This may be reasonable for a stable branch, but is not something I have any interest in across different release cycles. IMHO major VM upgrades should be considered the same as real hardware or firmware upgrades.

I think major VM upgrades is something that can be considered
occasionally but every 6 months would be pretty painful to most users. I think we ought to make a best effort to maintain compatibility with
older versions as long as it's reasonable.

Supporting live migration between different versions of qemu is a stretch goal. In the past 2 month we fixed about 10 basic live migration issues and mainline keeps breaking ;(

Nevertheless, as Anthony states, guest ABI should be rock stable.
We should have mechanism (machine conf format++) that will enable us
to configure pci addresses, cpuid entries, memory layout, device existence (hpet, virtio-console,..), etc.

Anyhow, I did sent a patch to allow virtio block to parametrized class value -
There wasn't a perfect match with finding the right interface to change it, happy to hear
other alternatives if you're not happy with it.



Anthony Liguori


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