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[Xen-devel] Question re live migrate on Xen 4.2 re different cpu capabilities



We've run into an issue on live migrate on Xen 4.2. We've mainly tested HVM on qemu-upstream DM with my live migrate patches in, but it seems to apply in any HVM live migrate.

What we see is under certain circumstances a linux guest booted on machine A will live-migrate to machine B and back again, but a linux guest booted on machine B will not live migrate to machine A - it hangs after the migration.

Tracking this down to different pairs of machines A and B, the difference seems to be down to different CPU capabilities. In particular, if B has identical CPU flags to A except that B supports xsave, the above seems to happen reliably. I presume what may be happening is that the guest notes and uses the presence of xsave on boot, and becomes unhappy when the recipient machine does not have it.

KVM with default settings does not suffer from this issue. I presume it is presenting by default a masked set of CPU capabilities.

Is there any way to do similarly in Xen? Or is Xen live migrate effectively restricted to live migrate between identical CPUs?

--
Alex Bligh

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