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Re: [Qemu-devel] [RFC] More robust migration


From: Anthony Liguori
Subject: Re: [Qemu-devel] [RFC] More robust migration
Date: Mon, 23 Feb 2009 19:15:14 -0600
User-agent: Thunderbird 2.0.0.19 (X11/20090105)

Jamie Lokier wrote:
Paul Brook wrote:
I never said you need the same host. All the save/restore code
should be host independent. It should be possible to save state on
(say) i386 and restore on ppc64. Anything that prevents this is IMO
a bug.

For KVM you're likely to need a cpu with at least as many features
as the old one, but that's the price you pay for using host hardware
features.

I'd prefer the "host hardware features" to be an acceleration
mechanism, than something which makes a VM dependent on the specific
host it's running on.

Can't KVM invoke QEMU's emulation capabilities for those things it
cannot provide itself because of missing host abilities?

In theory, software can do anything :-) In practice, it's pretty nasty to mix TCG execution with direct execution. TCG doesn't guarantee atomicity of translated instructions so if you had TCG running one VCPU and bare metal running the other badness could occur. Plus, TCG has hidden state that needs to be synchronized between the two.

Yeah, you could halt all VCPUs and just run one in TCG, or some other equally hackish thing. But the point is, it's not easy and since it's not a tremendously popular thing to do, noone's working on it. We have much more important low-hanging fruit (like making qcow2 not suck).

Regards,

Anthony Liguori

-- Jamie







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