[xen master] x86/mwait-idle: clean up BYT/CHT auto demotion disable

[patchbot@xxxxxxx]

commit 68b307f90a5739ed512946f42f716383a58dc4c0
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Mon May 18 08:55:06 2026 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon May 18 08:55:06 2026 +0200

    x86/mwait-idle: clean up BYT/CHT auto demotion disable
    
    Bay Trail (BYT) and Cherry Trail (CHT) platforms have a very specific way
    of disabling auto-demotion via specific MSR bits. Clean up the code so that
    BYT/CHT-specifics do not show up in the common 'struct idle_cpu' data
    structure.
    
    Remove the 'byt_auto_demotion_disable_flag' flag from 'struct idle_cpu',
    because a better coding pattern is to avoid very case-specific fields like
    'bool byt_auto_demotion_disable_flag' in a common data structure, which is
    used for all platforms, not only BYT/CHT. The code is just more readable
    when common data structures contain only commonly used fields.
    
    Instead, match BYT/CHT in the 'intel_idle_init_cstates_icpu()' function,
    and introduce a small helper to take care of BYT/CHT auto-demotion. This
    is consistent with how platform-specific things are done for other
    platforms.
    
    No intended functional changes.
    
    Inspired by (and description largely taken from) Linux'es c93d13b661a6
    ("intel_idle: clean up BYT/CHT auto demotion disable").
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
 xen/arch/x86/cpu/mwait-idle.c | 27 +++++++++++++++++----------
 1 file changed, 17 insertions(+), 10 deletions(-)

diff --git a/xen/arch/x86/cpu/mwait-idle.c b/xen/arch/x86/cpu/mwait-idle.c
index d575a256a4..8982b28543 100644
--- a/xen/arch/x86/cpu/mwait-idle.c
+++ b/xen/arch/x86/cpu/mwait-idle.c
@@ -103,7 +103,6 @@ struct idle_cpu {
         * Indicate which enable bits to clear here.
         */
        unsigned long auto_demotion_disable_flags;
-       bool byt_auto_demotion_disable_flag;
        enum c1e_promotion c1e_promotion;
 };
 
@@ -1144,7 +1143,7 @@ static void cf_check auto_demotion_disable(void *dummy)
        wrmsrl(MSR_PKG_CST_CONFIG_CONTROL, msr_bits);
 }
 
-static void cf_check byt_auto_demotion_disable(void *dummy)
+static void byt_cht_auto_demotion_disable(void)
 {
        wrmsrl(MSR_CC6_DEMOTION_POLICY_CONFIG, 0);
        wrmsrl(MSR_MC6_DEMOTION_POLICY_CONFIG, 0);
@@ -1195,13 +1194,11 @@ static const struct idle_cpu idle_cpu_snb = {
 static const struct idle_cpu idle_cpu_byt = {
        .state_table = byt_cstates,
        .c1e_promotion = C1E_PROMOTION_DISABLE,
-       .byt_auto_demotion_disable_flag = true,
 };
 
 static const struct idle_cpu idle_cpu_cht = {
        .state_table = cht_cstates,
        .c1e_promotion = C1E_PROMOTION_DISABLE,
-       .byt_auto_demotion_disable_flag = true,
 };
 
 static const struct idle_cpu idle_cpu_ivb = {
@@ -1677,14 +1674,11 @@ static int __init mwait_idle_probe(void)
        return 0;
 }
 
-static void mwait_idle_cpu_tweak(unsigned int cpu)
+static void mwait_idle_cpu_tweak(unsigned int cpu, bool bsp)
 {
        if (icpu->auto_demotion_disable_flags)
                on_selected_cpus(cpumask_of(cpu), auto_demotion_disable, NULL, 
1);
 
-       if (icpu->byt_auto_demotion_disable_flag)
-               on_selected_cpus(cpumask_of(cpu), byt_auto_demotion_disable, 
NULL, 1);
-
        switch (icpu->c1e_promotion) {
        case C1E_PROMOTION_DISABLE:
                on_selected_cpus(cpumask_of(cpu), c1e_promotion_disable, NULL, 
1);
@@ -1697,12 +1691,24 @@ static void mwait_idle_cpu_tweak(unsigned int cpu)
        case C1E_PROMOTION_PRESERVE:
                break;
        }
+
+       /* Pkg-scope MSRs on 1-socket-only systems need writing only once. */
+       if (!bsp)
+               return;
+
+       switch (boot_cpu_data.vfm) {
+       case INTEL_ATOM_SILVERMONT:
+       case INTEL_ATOM_AIRMONT:
+               byt_cht_auto_demotion_disable();
+               break;
+       }
 }
 
 static int cf_check mwait_idle_cpu_init(
     struct notifier_block *nfb, unsigned long action, void *hcpu)
 {
        unsigned int cpu = (unsigned long)hcpu, cstate;
+       static bool first = true;
        struct acpi_processor_power *dev = processor_powers[cpu];
 
        switch (action) {
@@ -1781,7 +1787,8 @@ static int cf_check mwait_idle_cpu_init(
                dev->count++;
        }
 
-       mwait_idle_cpu_tweak(cpu);
+       mwait_idle_cpu_tweak(cpu, first);
+       first = false;
 
        return NOTIFY_DONE;
 }
@@ -1818,7 +1825,7 @@ void mwait_idle_resume(void)
        if (!icpu)
                return;
 
-       mwait_idle_cpu_tweak(smp_processor_id());
+       mwait_idle_cpu_tweak(smp_processor_id(), true);
 }
 
 /* Helper function for HPET. */
--
generated by git-patchbot for /home/xen/git/xen.git#master