KVM: Initialize fpu state in preemptible context

init_fpu() (which is indirectly called by the fpu switching code) assumes
it is in process context.  Rather than makeing init_fpu() use an atomic
allocation, which can cause a task to be killed, make sure the fpu is
already initialized when we enter the run loop.

KVM-Stable-Tag.
Reported-and-tested-by: Kirill A. Shutemov <kas@openvz.org>
Acked-by: Pekka Enberg <penberg@kernel.org>
Reviewed-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kernel/i387.c

@@ -169,6 +169,7 @@ int init_fpu(struct task_struct *tsk)
 	set_stopped_child_used_math(tsk);
 	return 0;
 }
+EXPORT_SYMBOL_GPL(init_fpu);
 
 /*
  * The xstateregs_active() routine is the same as the fpregs_active() routine,

arch/x86/kvm/x86.c

@@ -5376,6 +5376,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 	int r;
 	sigset_t sigsaved;
 
+	if (!tsk_used_math(current) && init_fpu(current))
+		return -ENOMEM;
+
 	if (vcpu->sigset_active)
 		sigprocmask(SIG_SETMASK, &vcpu->sigset, &sigsaved);