Il 10/04/2013 12:08, Gleb Natapov ha scritto: >> > What is the opinion from the KVM folks on this? Shall we start to >> > emulate instructions the host does not provide? In this particular case >> > a relatively simple patch fixes a problem (starting Atom optimized >> > kernels on non-Atom machines). > We can add the emulation, but we should not start announcing the instruction > availability to a guest if host cpu does not have it by default. This > may trick a guest into thinking that movbe is the fastest way to do > something when it is not. > This does highlight a weakness in CPU_GET_SUPPORTED_CPUID, but I think this is not a problem in practice. With a management layer such as oVirt it's not a problem. For example, oVirt has its own library of processors. It doesn't care if KVM enables movbe. If you tell it your datacenter is a mix of Haswells and Sandy Bridges it will pick the CPUID bits that are common to all. However, even without a suitable management layer it is also not really a problem. The only processors that support MOVBE are Atom and Haswell. Haswell adds a whole lot of extra CPUID features, hence "-cpu Haswell,enforce" will fail with or without movbe emulation. "-cpu Haswell" will disable all Haswell new features except movbe will remain slow; that's fine, I think, anyway it's not what you'ld do except to play with CPU models. Atom is not defined by QEMU; even if it was, it is unlikely to be specified for a KVM guest since Atom doesn't support hardware virtualization itself. The next AMD processor that has MOVBE will probably have at least another feature that is not in Opteron_G5, thus it will be the same as Haswell. Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
