After commit 77b0f5d6 (KVM: nVMX: Ack and write vector info to intr_info
if L1 asks us to), "Acknowledge interrupt on exit" behavior can be
emulated. To do so, KVM will ask the APIC for the interrupt vector if
during a nested vmexit if VM_EXIT_ACK_INTR_ON_EXIT is set.  With APICv,
kvm_get_apic_interrupt would return -1 and give the following WARNING:

Call Trace:
 [<ffffffff81493563>] dump_stack+0x49/0x5e
 [<ffffffff8103f0eb>] warn_slowpath_common+0x7c/0x96
 [<ffffffffa059709a>] ? nested_vmx_vmexit+0xa4/0x233 [kvm_intel]
 [<ffffffff8103f11a>] warn_slowpath_null+0x15/0x17
 [<ffffffffa059709a>] nested_vmx_vmexit+0xa4/0x233 [kvm_intel]
 [<ffffffffa0594295>] ? nested_vmx_exit_handled+0x6a/0x39e [kvm_intel]
 [<ffffffffa0537931>] ? kvm_apic_has_interrupt+0x80/0xd5 [kvm]
 [<ffffffffa05972ec>] vmx_check_nested_events+0xc3/0xd3 [kvm_intel]
 [<ffffffffa051ebe9>] inject_pending_event+0xd0/0x16e [kvm]
 [<ffffffffa051efa0>] vcpu_enter_guest+0x319/0x704 [kvm]

To fix this, we cannot rely on the processor's virtual interrupt delivery,
because "acknowledge interrupt on exit" must only update the virtual
ISR/PPR/IRR registers (and SVI, which is just a cache of the virtual ISR)
but it should not deliver the interrupt through the IDT.  Thus, KVM has
to deliver the interrupt "by hand", similar to the treatment of EOI in
commit fc57ac2c (KVM: lapic: sync highest ISR to hardware apic on
EOI, 2014-05-14).

The patch modifies kvm_cpu_get_interrupt to always acknowledge an
interrupt; there are only two callers, and the other is not affected
because it is never reached with kvm_apic_vid_enabled() == true.  Then it
modifies apic_set_isr and apic_clear_irr to update SVI and RVI in addition
to the registers.
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Suggested-by: "Zhang, Yang Z" <yang.z.zhang@intel.com>
Tested-by: Liu, RongrongX <rongrongx.liu@intel.com>
Tested-by: Felipe Reyes <freyes@suse.com>
Fixes: 77b0f5d6
Cc: stable@vger.kernel.org
Signed-off-by: Wanpeng Li <wanpeng.li@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>