[PATCH v4] x86/mwait-idle: re-order state entry/exit code a little

[Jan Beulich <jbeulich@xxxxxxxx>]

The initial observation is that unlike the original ACPI idle driver we
have a 2nd cpu_is_haltable() in here. By making the actual state entry
conditional, the emitted trace records as well as the subsequent stats
update are at least misleading in case the state wasn't actually entered.
Hence they would want moving inside the conditional. At which point the
cpuidle_get_tick() invocations could (and hence should) move as well.
cstate_restore_tsc() also isn't needed if we didn't actually enter the
state.

This leaves only the errata_c6_workaround() and lapic_timer_off()
invocations outside the conditional. As a result it looks easier to
drop the conditional (and come back in sync with the other driver again)
than to move almost everything into the conditional.

While there also move the TRACE_6D() out of the IRQ-disabled region.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Moving the TRACE_6D() may be a little controversial, as this could lead
to a sequence of trace records not actually representing the sequence of
events, in case further records get emitted by interrupt handlers. But
with us now conditionally enabling interrupts around MWAIT, that issue
exists already anyway.

Unlike said in the earlier outline of the alternative approach,
errata_c6_workaround() cannot be moved: cpu_has_pending_apic_eoi() needs
to be called when IRQs are already off.
---
v4: Different approach (and title), as was previously outlined as an
    alternative.
v3: Also move cstate_restore_tsc() invocation and split ones to
    update_idle_stats().
v2: New.

--- a/xen/arch/x86/cpu/mwait-idle.c
+++ b/xen/arch/x86/cpu/mwait-idle.c
@@ -847,26 +847,25 @@ static void mwait_idle(void)
 
        update_last_cx_stat(power, cx, before);
 
-       if (cpu_is_haltable(cpu)) {
-               if (cx->irq_enable_early)
-                       local_irq_enable();
+       if (cx->irq_enable_early)
+               local_irq_enable();
 
-               mwait_idle_with_hints(cx->address, MWAIT_ECX_INTERRUPT_BREAK);
+       mwait_idle_with_hints(cx->address, MWAIT_ECX_INTERRUPT_BREAK);
 
-               local_irq_disable();
-       }
+       local_irq_disable();
 
        after = alternative_call(cpuidle_get_tick);
 
        cstate_restore_tsc();
        trace_exit_reason(irq_traced);
-       TRACE_6D(TRC_PM_IDLE_EXIT, cx->type, after,
-               irq_traced[0], irq_traced[1], irq_traced[2], irq_traced[3]);
 
        /* Now back in C0. */
        update_idle_stats(power, cx, before, after);
        local_irq_enable();
 
+       TRACE_6D(TRC_PM_IDLE_EXIT, cx->type, after,
+               irq_traced[0], irq_traced[1], irq_traced[2], irq_traced[3]);
+
        if (!(lapic_timer_reliable_states & (1 << cx->type)))
                lapic_timer_on();