[patch 1/4] x86/cpu: Ensure that CPU info updates are propagated on UP

From: Thomas Gleixner
Date: Fri Mar 22 2024 - 14:56:53 EST


The boot sequence evaluates CPUID information twice:

1) During early boot

2) When finalizing the early setup right before
mitigations are selected and alternatives are patched.

In both cases the evaluation is stored in boot_cpu_data, but on UP the
copying of boot_cpu_data to the per CPU info of the boot CPU happens
between #1 and #2. So any update which happens in #2 is never propagated to
the per CPU info instance.

Consolidate the whole logic and copy boot_cpu_data right before applying
alternatives as that's the point where boot_cpu_data is in it's final state
and not supposed to change anymore.

This also removes the voodoo mb() from smp_prepare_cpus_common() which had
absolutely no purpose.

Fixes: 71eb4893cfaf ("x86/percpu: Cure per CPU madness on UP")
Reported-by: Guenter Roeck <linux@xxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
arch/x86/kernel/cpu/common.c | 9 +++++++++
arch/x86/kernel/setup.c | 10 ----------
arch/x86/kernel/smpboot.c | 32 +++++---------------------------
3 files changed, 14 insertions(+), 37 deletions(-)

--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2307,6 +2307,8 @@ void arch_smt_update(void)

void __init arch_cpu_finalize_init(void)
{
+ struct cpuinfo_x86 *c = this_cpu_ptr(&cpu_info);
+
identify_boot_cpu();

select_idle_routine();
@@ -2345,6 +2347,13 @@ void __init arch_cpu_finalize_init(void)
fpu__init_system();
fpu__init_cpu();

+ /*
+ * Ensure that access to the per CPU representation has the initial
+ * boot CPU configuration.
+ */
+ *c = boot_cpu_data;
+ c->initialized = true;
+
alternative_instructions();

if (IS_ENABLED(CONFIG_X86_64)) {
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -1206,16 +1206,6 @@ void __init i386_reserve_resources(void)

#endif /* CONFIG_X86_32 */

-#ifndef CONFIG_SMP
-void __init smp_prepare_boot_cpu(void)
-{
- struct cpuinfo_x86 *c = &cpu_data(0);
-
- *c = boot_cpu_data;
- c->initialized = true;
-}
-#endif
-
static struct notifier_block kernel_offset_notifier = {
.notifier_call = dump_kernel_offset
};
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -313,14 +313,6 @@ static void notrace start_secondary(void
cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
}

-static void __init smp_store_boot_cpu_info(void)
-{
- struct cpuinfo_x86 *c = &cpu_data(0);
-
- *c = boot_cpu_data;
- c->initialized = true;
-}
-
/*
* The bootstrap kernel entry code has set these up. Save them for
* a given CPU
@@ -1039,29 +1031,15 @@ static __init void disable_smp(void)
cpumask_set_cpu(0, topology_die_cpumask(0));
}

-static void __init smp_cpu_index_default(void)
-{
- int i;
- struct cpuinfo_x86 *c;
-
- for_each_possible_cpu(i) {
- c = &cpu_data(i);
- /* mark all to hotplug */
- c->cpu_index = nr_cpu_ids;
- }
-}
-
void __init smp_prepare_cpus_common(void)
{
unsigned int i;

- smp_cpu_index_default();
-
- /*
- * Setup boot CPU information
- */
- smp_store_boot_cpu_info(); /* Final full version of the data */
- mb();
+ /* Mark all except the boot CPU as hotpluggable */
+ for_each_possible_cpu(i) {
+ if (i)
+ per_cpu(cpu_info.cpu_index, i) = nr_cpu_ids;
+ }

for_each_possible_cpu(i) {
zalloc_cpumask_var(&per_cpu(cpu_sibling_map, i), GFP_KERNEL);