[xen stable-4.18] Revert part of "x86/mwait-idle: disable IBRS during long idle"

[patchbot@xxxxxxx]

commit 43c513c3197d6bed727c82cddd471afc8be0bbd4
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue Jun 24 15:20:52 2025 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Mon Jul 7 22:52:11 2025 +0100

    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: 08acdf9a2615 ("x86/mwait-idle: disable IBRS during long idle")
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    (cherry picked from commit 07d7163334a7507d329958b19d976be769580999)
---
 xen/arch/x86/cpu/mwait-idle.c | 12 ++----------
 1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/xen/arch/x86/cpu/mwait-idle.c b/xen/arch/x86/cpu/mwait-idle.c
index ff5c808bc9..e95fe5d889 100644
--- a/xen/arch/x86/cpu/mwait-idle.c
+++ b/xen/arch/x86/cpu/mwait-idle.c
@@ -891,7 +891,6 @@ 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;
@@ -918,6 +917,8 @@ 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);
@@ -944,11 +945,6 @@ 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
@@ -980,10 +976,6 @@ 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,
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.18