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.

Fixes: b41f8638 ("KVM: VMX: Isolate pure loads from atomic XCHG when processing PIR")
Reported-by: Farrah Chen <farrah.chen@intel.com>
Analyzed-by: Chenyi Qiang <chenyi.qiang@intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/kvm/20260428070349.1633238-1-chenyi.qiang@intel.com/T/
Link: https://patch.msgid.link/20260503201703.108231-2-pbonzini@redhat.com/


Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>