Comment 30 for bug 1561019

"-cpu Westmere,+avx" actually should enable the bit on CPUID if and only if KVM is able to handle the feature. When KVM can't handle the feature, it should be filtered out before the guest CPUID table is built. I still don't understand why exactly the guest got an invalid operation exception, as the instruction was supposed to be working. Maybe it's related to the "level" field and the xsave feature (that is required for AVX, as far as I recall), that needs level >= 0xD.

I don't know if the guest is really allowed to use the feature when the AVX bit is set but the necessary xsave bits are not present (in it is not, then this is a guest bug). If the guest was simply misled by the CPUID information, and correct in trying to use the instructions, it is a QEMU bug (QEMU should have disabled the feature, and abort in case the "enforce" flag is set). On either case, it is not a libvirt bug to ask for "-cpu Westmere,+avx".

But it would be interesting if libvirt could treat some CPU features as "can be safely disabled". It would be much better if libvirt used "-cpu SandyBridge,-x2apic" on that host, instead of "-cpu Westmere,+<lots of flags>".