KVM: x86: Unconditionally enable irqs in guest context

On VMX, KVM currently does not re-enable irqs until after it has exited
the guest context.  As a result, a tick that fires in the window between
VM-Exit and guest_exit_irqoff() will be accounted as system time.  While
said window is relatively small, it's large enough to be problematic in
some configurations, e.g. if VM-Exits are consistently occurring a hair
earlier than the tick irq.

Intentionally toggle irqs back off so that guest_exit_irqoff() can be
used in lieu of guest_exit() in order to avoid the save/restore of flags
in guest_exit().  On my Haswell system, "nop; cli; sti" is ~6 cycles,
versus ~28 cycles for "pushf; pop <reg>; cli; push <reg>; popf".

Fixes: f2485b3e0c ("KVM: x86: use guest_exit_irqoff")
Reported-by: Wei Yang <w90p710@gmail.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -8046,7 +8046,18 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 
 	kvm_x86_ops->handle_exit_irqoff(vcpu);
 
+	/*
+	 * Consume any pending interrupts, including the possible source of
+	 * VM-Exit on SVM and any ticks that occur between VM-Exit and now.
+	 * An instruction is required after local_irq_enable() to fully unblock
+	 * interrupts on processors that implement an interrupt shadow, the
+	 * stat.exits increment will do nicely.
+	 */
+	kvm_before_interrupt(vcpu);
+	local_irq_enable();
 	++vcpu->stat.exits;
+	local_irq_disable();
+	kvm_after_interrupt(vcpu);
 
 	guest_exit_irqoff();
 	if (lapic_in_kernel(vcpu)) {