On 14/12/2014 15:31, Nadav Amit wrote: > This behaviour hurts the “equivalence property” - the VM does not behave as > bare-metal system. Moreover, it may hurt the VM performance if the VM uses > XSAVEOPT (and not XSAVES), has MXCSR with value different than the reset > value of 1F80H and has all SSE registers set to zero. In such case, the VM > would save/restore SSE registers unnecessarily. I don’t know whether such > scenario happens in real workloads. > > tl;dr - hypervisors which use XSAVES (and XSAVEC) mess the VM state and may > hurt VM performance. Perhaps KVM should use XSAVE/XSAVEOPT instead. Unfortunately KVM uses whatever the kernel uses. KVM does not save/restore itself the FPU state. The same problem above can happen if the kernel uses XSAVES/XRSTORS and userspace uses XSAVEOPT. So it's a processor bug. 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
