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KVM guests must not be allowed to update the host CPU microcode. KVM does not allows this and instead always returns the same microcode revision or patch level value to the guest. If the guest tries to update the CPU microcode, it will fail and show an error message similar to:
CPU0: update failed (for patch_level=0x6000624)
To work around this, configure the guest to not install CPU microcode updates; for example, uninstall the microcode_ctl package Red Hat Enterprise Linux of Fedora guests.
A microcode update on the virtual CPU doesn't even make sense, the guest shouldn't try to update it. We just return a dummy value on the MSR_IA32_UCODE_REV (== MSR_AMD64_PATCH_LEVEL == 0x0000008b) MSR to make some guests happier.
One thing we can consider implementing upstream is to make the patch level value configurable on the QEMU CPU model table, to make a bogus microcode update error message less likely, but that's probably not a feature for RHEL-6.