Revert part of "x86/mwait-idle: disable IBRS during long idle"

Most of the patch (handling of CPUIDLE_FLAG_IBRS) is fine, but the
adjustements to mwait_idle() are not; spec_ctrl_enter_idle() does more than
just alter MSR_SPEC_CTRL.IBRS.

The only reason this doesn't need an XSA is because the unconditional
spec_ctrl_{enter,exit}_idle() in mwait_idle_with_hints() were left unaltered,
and thus the MWAIT remained properly protected.

There (would have been) two problems.  In the ibrs_disable (== deep C) case:

 * On entry, VERW and RSB-stuffing are architecturally skipped.
 * On exit, there's a branch crossing the WRMSR which reinstates the
   speculative safety for indirect branches.

All this change did was double up the expensive operations in the deep C case,
and fail to optimise the intended case.

I have an idea of how to plumb this more nicely, but it requires larger
changes to legacy IBRS handling to not make spec_ctrl_enter_idle() vulnerable
in other ways.  In the short term, simply take out the perf hit.

Fixes: 08acdf9a26 ("x86/mwait-idle: disable IBRS during long idle")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 07d7163334)

xen/arch/x86/cpu/mwait-idle.c

@@ -902,7 +902,6 @@ static const struct cpuidle_state snr_cstates[] = {
 static void cf_check mwait_idle(void)
 {
 	unsigned int cpu = smp_processor_id();
-	struct cpu_info *info = get_cpu_info();
 	struct acpi_processor_power *power = processor_powers[cpu];
 	struct acpi_processor_cx *cx = NULL;
 	unsigned int next_state;
@@ -929,6 +928,8 @@ static void cf_check mwait_idle(void)
 			pm_idle_save();
 		else
 		{
+			struct cpu_info *info = get_cpu_info();
+
 			spec_ctrl_enter_idle(info);
 			safe_halt();
 			spec_ctrl_exit_idle(info);
@@ -955,11 +956,6 @@ static void cf_check mwait_idle(void)
 	if ((cx->type >= 3) && errata_c6_workaround())
 		cx = power->safe_state;
 
-	if (cx->ibrs_disable) {
-		ASSERT(!cx->irq_enable_early);
-		spec_ctrl_enter_idle(info);
-	}
-
 #if 0 /* XXX Can we/do we need to do something similar on Xen? */
 	/*
 	 * leave_mm() to avoid costly and often unnecessary wakeups
@@ -991,10 +987,6 @@ static void cf_check mwait_idle(void)
 
 	/* Now back in C0. */
 	update_idle_stats(power, cx, before, after);
-
-	if (cx->ibrs_disable)
-		spec_ctrl_exit_idle(info);
-
 	local_irq_enable();
 
 	TRACE_6D(TRC_PM_IDLE_EXIT, cx->type, after,