KVM: x86: Add a WARN on TIF_NEED_FPU_LOAD in kvm_load_guest_fpu()

WARN once in kvm_load_guest_fpu() if TIF_NEED_FPU_LOAD is observed, as
that would mean that KVM is corrupting userspace's FPU by saving
unknown register state into arch.user_fpu.  Add a comment to explain
why KVM WARNs on TIF_NEED_FPU_LOAD instead of implementing logic
similar to fpu__copy().
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -8476,6 +8476,13 @@ static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu)
 {
 	fpregs_lock();
 
+	/*
+	 * Reloading userspace's FPU is handled by kvm_arch_vcpu_load(), both
+	 * for direct calls from userspace (via vcpu_load()) and if this task
+	 * is preempted (via kvm_sched_in()) between vcpu_load() and now.
+	 */
+	WARN_ON_ONCE(test_thread_flag(TIF_NEED_FPU_LOAD));
+
 	copy_fpregs_to_fpstate(vcpu->arch.user_fpu);
 	/* PKRU is separately restored in kvm_x86_ops->run.  */
 	__copy_kernel_to_fpregs(&vcpu->arch.guest_fpu->state,