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[hyperv] Do not steal ownership from the Gen 2 UEFI firmware
We must not steal ownership from the Gen 2 UEFI firmware, since doing so will cause an immediate system crash (most likely in the form of a reboot). This problem was masked before commit a0f6e75 ("[hyperv] Do not fail if guest OS ID MSR is already set"), since prior to that commit we would always fail if we found any non-zero guest OS identity. We now accept a non-zero previous guest OS identity in order to allow for situations such as chainloading from iPXE to another iPXE, and as a prerequisite for commit b91cc98 ("[hyperv] Cope with Windows Server 2016 enlightenments"). A proper fix would be to reverse engineer the UEFI protocols exposed within the Hyper-V Gen 2 firmware and use these to bind to the VMBus device representing the network connection, (with the native Hyper-V driver moved to become a BIOS-only feature). As an interim solution, fail to initialise the native Hyper-V driver if we detect the guest OS identity known to be used by the Gen 2 UEFI firmware. This will cause the standard all-drivers build (ipxe.efi) to fall back to using the SNP driver. Signed-off-by: Michael Brown <email@example.com>
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