xen-kernel -- information leak in legacy x86 FPU/XMM initialization

The Xen Project reports:

When XSAVE/XRSTOR are not in use by Xen to manage guest extended
register state, the initial values in the FPU stack and XMM
registers seen by the guest upon first use are those left there by
the previous user of those registers.
A malicious domain may be able to leverage this to obtain sensitive
information such as cryptographic keys from another domain.