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>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0af5cb637beaa7bf345cf93ea44701e8a44488a1..25aac4c81b1242c85951e96f0a95a0ee024c7129 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/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,