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Red Hat & AMD Demo Live VM Migration Across CPU Vendors
An anonymous reader notes an Inquirer story reporting on something of a breakthrough in virtual machine management — a demonstration (not yet a product) of migrating a running virtual machine across CPUs from different vendors (video here). "Red Hat and AMD have just done the so called impossible, and demonstrated VM live migration across CPU architectures. Not only that, they have demonstrated it across CPU vendors, potentially commoditizing server processors. This is quite a feat. Only a few months ago during VMworld, Intel and VMware claimed that this was impossible. Judging by an initial response, VMware is quite irked by this KVM accomplishment and they are pointing to stability concerns. This sound like scaremongering to me ... All the interesting controversy aside, cross-vendor migration is [obviously] a good thing for customers because it avoids platform lock-in."
Open source is for morons.
Only Apple has the engineering know-how and skills to pull of something like this. The fact that they have not done so to date is a clear indication that it is impossible.
Go to 4:05 in the video. On the far left, you can see from the blue intel line that the guest is running there, then they migrate, and the blue line goes to the idle point, and the orange line starts taking the load. But NOTICE, the AMD line is consistantly higher than the intel line was. I'm no intel fanboy... or AMD. I have both intel and amd servers in my racks. I just thought it was interesting, and I'm surprised they let the video go out like that.
Do not meddle in the affairs of sysadmins, for they are subtle, and quick to anger.
The new Intel/AMD CPU features that allow masking of CPUID bits while running virtualized also make processors recent enough that most of the interesting features are present - MMX, SSE up to ~3. The "common subset" ends up looking like an early Core2 or a Barcelona (minus the VT/SVM feature bits, of course) - Intel and AMD run about a generation behind on adding each other's instructions. Run on anything older than the latest processors, and you have to trap-and-emulate every CPUID instruction. Enough code still uses CPUID as a serializing instruction that this has noticeable overhead.
So there are two strategies. Pass directly through the CPUID bits (and on the newest processors, apply a mask), or remember a baseline value, trap-and-emulate every CPUID and always return that value. Sounds like KVM has picked the latter approach for a default; VMware's default is to expose the actual processor features and accept a mask as an optional override, which skews towards exposing more features at the expense of some compatibility. Equally valid choices, IMHO.
The Worst Case Scenario when not doing a trap-and-emulate of every CPUID is an app that does CPUID, reads the vendor string, then decides based on the vendor string which other CPUID leafs to read. (Like the 0x80000000 leafs, which are vendor-specific and would come back as gibberish if you get the processor wrong). If the app migrates during the dozen or so instructions between the first CPUID and the following ones, instant corruption. Good enough for a pretty demo, destined to make a guest kernel die a few times a year if actually used in production. And I'm 95% sure this is what the OP demo is doing - living dangerously by hoping mismatched CPUID results never get noticed.
I agree with Anthony Liguori here - on a production machine, an Intel/AMD migration is way too much of a stupid risk. All you have to do is reboot the VM, it's much safer.
(As a side note to everyone reading, the reason Linux timekeeping is such a problem is that TSC issue. Intel long ago stated TSC was NOT supposed to be used as a timesource. Linux kernel folks ignored the warning, made non-virtualizable assumptions, and today are in a world of hurt for timekeeping in a VM. And only now, many years later, are patching the kernel to detect hypervisors to work around the problem.)
A witty [sig] proves nothing. --Voltaire