KVM: x86: Do IRR scan in __kvm_apic_update_irr even if PIR is empty

In the Linux kernel, the following vulnerability has been resolved:

KVM: x86: Do IRR scan in __kvm_apic_update_irr even if PIR is empty

Fall back to apic_find_highest_vector() when PID.ON is set but PIR
turns out to be empty, to correctly report the highest pending interrupt
from the existing IRR.

In a nested VM stress test, the following WARNING fires in
vmx_check_nested_events() when kvm_cpu_has_interrupt() reports a pending
interrupt but the subsequent kvm_apic_has_interrupt() (which invokes
vmx_sync_pir_to_irr() again) returns -1:

WARNING: CPU: 99 PID: 57767 at arch/x86/kvm/vmx/nested.c:4449 vmx_check_nested_events+0x6bf/0x6e0 [kvm_intel]
Call Trace:
kvm_check_and_inject_events
vcpu_enter_guest.constprop.0
vcpu_run
kvm_arch_vcpu_ioctl_run
kvm_vcpu_ioctl
__x64_sys_ioctl
do_syscall_64
entry_SYSCALL_64_after_hwframe

The root cause is a race between vmx_sync_pir_to_irr() on the target vCPU
and __vmx_deliver_posted_interrupt() on a sender vCPU. The sender
performs two individually-atomic operations that are not a single
transaction:

1. pi_test_and_set_pir(vector) -- sets the PIR bit
2. pi_test_and_set_on() -- sets PID.ON

The following interleaving triggers the bug:

Sender vCPU (IPI): Target vCPU (1st sync_pir_to_irr):
B1: set PIR[vector]
A1: pi_clear_on()
A2: pi_harvest_pir() -> sees B1 bit
A3: xchg() -> consumes bit, PIR=0
(1st sync returns correct max_irr)
B2: set PID.ON = 1

Target vCPU (2nd sync_pir_to_irr):
C1: pi_test_on() -> TRUE (from B2)
C2: pi_clear_on() -> ON=0
C3: pi_harvest_pir() -> PIR empty
C4: *max_irr = -1, early return
IRR NOT SCANNED

The interrupt is not lost (it resides in the IRR from the first sync and
is recovered on the next vcpu_enter_guest() iteration), but the incorrect
max_irr causes a spurious WARNING and a wasted L2 VM-Enter/VM-Exit cycle.