[PATCH v2] KVM: nVMX: Fix attempting to emulate "Acknowledge interrupt on exit" when there is no interrupt which L1 requires to inject to L2

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpeng.li@hotmail.com>

------------[ cut here ]------------
 WARNING: CPU: 5 PID: 2288 at arch/x86/kvm/vmx.c:11124 nested_vmx_vmexit+0xd64/0xd70 [kvm_intel]
 CPU: 5 PID: 2288 Comm: qemu-system-x86 Not tainted 4.13.0-rc2+ #7
 RIP: 0010:nested_vmx_vmexit+0xd64/0xd70 [kvm_intel]
Call Trace:
  vmx_check_nested_events+0x131/0x1f0 [kvm_intel]
  ? vmx_check_nested_events+0x131/0x1f0 [kvm_intel]
  kvm_arch_vcpu_ioctl_run+0x5dd/0x1be0 [kvm]
  ? vmx_vcpu_load+0x1be/0x220 [kvm_intel]
  ? kvm_arch_vcpu_load+0x62/0x230 [kvm]
  kvm_vcpu_ioctl+0x340/0x700 [kvm]
  ? kvm_vcpu_ioctl+0x340/0x700 [kvm]
  ? __fget+0xfc/0x210
  do_vfs_ioctl+0xa4/0x6a0
  ? __fget+0x11d/0x210
  SyS_ioctl+0x79/0x90
  do_syscall_64+0x8f/0x750
  ? trace_hardirqs_on_thunk+0x1a/0x1c
  entry_SYSCALL64_slow_path+0x25/0x25

This can be reproduced by booting L1 guest w/ 'noapic' grub parameter, which 
means that tells the kernel to not make use of any IOAPICs that may be present 
in the system.

Actually external_intr variable in nested_vmx_vmexit() is the req_int_win 
variable passed from vcpu_enter_guest() which means that the L0's userspace 
requests an irq window. I observed the scenario (!kvm_cpu_has_interrupt(vcpu) &&
L0's userspace reqeusts an irq window) is true, so there is no interrupt which 
L1 requires to inject to L2, we should not attempt to emualte "Acknowledge 
interrupt on exit" for the irq window requirement in this scenario.

This patch fixes it by not attempt to emulate "Acknowledge interrupt on exit"  
if there is no L1 requirement to inject an interrupt to L2.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
v1 -> v2:
 * update patch description
 * check nested_exit_intr_ack_set() first 

 arch/x86/kvm/vmx.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 2737343..c5a0ab5 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -11118,8 +11118,9 @@ static void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 exit_reason,
 
 	vmx_switch_vmcs(vcpu, &vmx->vmcs01);
 
-	if ((exit_reason == EXIT_REASON_EXTERNAL_INTERRUPT)
-	    && nested_exit_intr_ack_set(vcpu)) {
+	if (nested_exit_intr_ack_set(vcpu) &&
+		exit_reason == EXIT_REASON_EXTERNAL_INTERRUPT &&
+		kvm_cpu_has_interrupt(vcpu)) {
 		int irq = kvm_cpu_get_interrupt(vcpu);
 		WARN_ON(irq < 0);
 		vmcs12->vm_exit_intr_info = irq |
-- 
2.7.4