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
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