When delivering a virtual interrupt, don't actually send a posted interrupt
if the target vCPU is also the currently running vCPU and is IN_GUEST_MODE,
in which case the interrupt is being sent from a VM-Exit fastpath and the
core run loop in vcpu_enter_guest() will manually move the interrupt from
the PIR to vmcs.GUEST_RVI.  IRQs are disabled while IN_GUEST_MODE, thus
there's no possibility of the virtual interrupt being sent from anything
other than KVM, i.e. KVM won't suppress a wake event from an IRQ handler
(see commit fdba608f, "KVM: VMX: Wake vCPU when delivering posted IRQ
even if vCPU == this vCPU").

Eliding the posted interrupt restores the performance provided by the
combination of commits 379a3c8e ("KVM: VMX: Optimize posted-interrupt
delivery for timer fastpath") and 26efe2fd ("KVM: VMX: Handle
preemption timer fastpath").

Thanks Sean for better comments.
Suggested-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Message-Id: <1643111979-36447-1-git-send-email-wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>