KVM: X86: Fix fpu state crash in kvm guest

The idea before commit 240c35a37 (which has just been reverted)
was that we have the following FPU states:

               userspace (QEMU)             guest
---------------------------------------------------------------------------
               processor                    vcpu->arch.guest_fpu
>>> KVM_RUN: kvm_load_guest_fpu
               vcpu->arch.user_fpu          processor
>>> preempt out
               vcpu->arch.user_fpu          current->thread.fpu
>>> preempt in
               vcpu->arch.user_fpu          processor
>>> back to userspace
>>> kvm_put_guest_fpu
               processor                    vcpu->arch.guest_fpu
---------------------------------------------------------------------------

With the new lazy model we want to get the state back to the processor
when schedule in from current->thread.fpu.

Reported-by: Thomas Lambertz <mail@thomaslambertz.de>
Reported-by: anthony <antdev66@gmail.com>
Tested-by: anthony <antdev66@gmail.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Thomas Lambertz <mail@thomaslambertz.de>
Cc: anthony <antdev66@gmail.com>
Cc: stable@vger.kernel.org
Fixes: 5f409e20b (x86/fpu: Defer FPU state load until return to userspace)
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
[Add a comment in front of the warning. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index cf2afdf8facff7587edb4be90fa835c92b736030..7eb56f8e2ea8df3d16dd1e290f73d2b39a27c4ce 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3306,6 +3306,10 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 
        kvm_x86_ops->vcpu_load(vcpu, cpu);
 
+       fpregs_assert_state_consistent();
+       if (test_thread_flag(TIF_NEED_FPU_LOAD))
+               switch_fpu_return();
+
        /* Apply any externally detected TSC adjustments (due to suspend) */
        if (unlikely(vcpu->arch.tsc_offset_adjustment)) {
                adjust_tsc_offset_host(vcpu, vcpu->arch.tsc_offset_adjustment);
@@ -7990,9 +7994,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
        trace_kvm_entry(vcpu->vcpu_id);
        guest_enter_irqoff();
 
-       fpregs_assert_state_consistent();
-       if (test_thread_flag(TIF_NEED_FPU_LOAD))
-               switch_fpu_return();
+       /* The preempt notifier should have taken care of the FPU already.  */
+       WARN_ON_ONCE(test_thread_flag(TIF_NEED_FPU_LOAD));
 
        if (unlikely(vcpu->arch.switch_db_regs)) {
                set_debugreg(0, 7);