KVM: VMX: Fix invalid guest state detection after task-switch emulation

This can be reproduced by EPT=1, unrestricted_guest=N, emulate_invalid_state=Y
or EPT=0, the trace of kvm-unit-tests/taskswitch2.flat is like below, it tries
to emulate invalid guest state task-switch:

kvm_exit: reason TASK_SWITCH rip 0x0 info 40000058 0
kvm_emulate_insn: 42000:0:0f 0b (0x2)
kvm_emulate_insn: 42000:0:0f 0b (0x2) failed
kvm_inj_exception: #UD (0x0)
kvm_entry: vcpu 0
kvm_exit: reason TASK_SWITCH rip 0x0 info 40000058 0
kvm_emulate_insn: 42000:0:0f 0b (0x2)
kvm_emulate_insn: 42000:0:0f 0b (0x2) failed
kvm_inj_exception: #UD (0x0)
......................

It appears that the task-switch emulation updates rflags (and vm86
flag) only after the segments are loaded, causing vmx->emulation_required
to be set, when in fact invalid guest state emulation is not needed.

This patch fixes it by updating vmx->emulation_required after the
rflags (and vm86 flag) is updated in task-switch emulation.

Thanks Radim for moving the update to vmx__set_flags and adding Paolo's
suggestion for the check.
Suggested-by: Nadav Amit <nadav.amit@gmail.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/vmx.c

@@ -2327,6 +2327,11 @@ static void vmx_vcpu_put(struct kvm_vcpu *vcpu)
 	__vmx_load_host_state(to_vmx(vcpu));
 }
 
+static bool emulation_required(struct kvm_vcpu *vcpu)
+{
+	return emulate_invalid_guest_state && !guest_state_valid(vcpu);
+}
+
 static void vmx_decache_cr0_guest_bits(struct kvm_vcpu *vcpu);
 
 /*
@@ -2364,6 +2369,8 @@ static unsigned long vmx_get_rflags(struct kvm_vcpu *vcpu)
 
 static void vmx_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
 {
+	unsigned long old_rflags = vmx_get_rflags(vcpu);
+
 	__set_bit(VCPU_EXREG_RFLAGS, (ulong *)&vcpu->arch.regs_avail);
 	to_vmx(vcpu)->rflags = rflags;
 	if (to_vmx(vcpu)->rmode.vm86_active) {
@@ -2371,6 +2378,9 @@ static void vmx_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
 		rflags |= X86_EFLAGS_IOPL | X86_EFLAGS_VM;
 	}
 	vmcs_writel(GUEST_RFLAGS, rflags);
+
+	if ((old_rflags ^ to_vmx(vcpu)->rflags) & X86_EFLAGS_VM)
+		to_vmx(vcpu)->emulation_required = emulation_required(vcpu);
 }
 
 static u32 vmx_get_pkru(struct kvm_vcpu *vcpu)
@@ -3858,11 +3868,6 @@ static __init int alloc_kvm_area(void)
 	return 0;
 }
 
-static bool emulation_required(struct kvm_vcpu *vcpu)
-{
-	return emulate_invalid_guest_state && !guest_state_valid(vcpu);
-}
-
 static void fix_pmode_seg(struct kvm_vcpu *vcpu, int seg,
 		struct kvm_segment *save)
 {