I have not been able to trigger this bug on Linux 4.3, and suspect
it is due to this commit from Linux 4.2:
653f52c kvm,x86: load guest FPU context more eagerly
With this commit, as long as the host is using eagerfpu, the guest's
fpu is always loaded just before the guest's xcr0 (vcpu->fpu_active
is always 1 in the following snippet):
6569 if (vcpu->fpu_active)
When the guest's fpu is loaded, irq_fpu_usable() returns false.
We've included our workaround for this bug, which applies to Linux 3.11.
It does not apply cleanly to HEAD since the fpu subsystem was refactored
in Linux 4.2. While the latest kernel does not look vulnerable, we may
want to apply a fix to the vulnerable stable kernels.