Hi Tobin,<br /><br />as mentioned in the reply to the QEMU patches posted by Tobin, I think the firmware helper approach is very good, but there are some disadvantages in the idea of auxiliary vCPUs. These are especially true in the VMM, where it's much nicer to have a separate VM that goes through a specialized run loop; however, even in the firmware level there are some complications (as you pointed out) in letting MpService workers run after ExitBootServices.<br /><br />My idea would be that the firmware would start the VM as usual using the same launch data; then, the firmware would detect it was running as a migration helper VM during the SEC or PEI phases (for example via the GHCB or some other unencrypted communication area), and divert execution to the migration helper instead of proceeding to the next boot phase. This would be somewhat similar in spirit to how edk2 performs S3 resume, if my memory serves correctly.<br /><br />What do you think?<br /><br />Thanks,<br /><br />Paolo