x86/percpu: Cure per CPU madness on UP

On UP builds Sparse complains rightfully about accesses to cpu_info with
per CPU accessors:

  cacheinfo.c:282:30: sparse: warning: incorrect type in initializer (different address spaces)
  cacheinfo.c:282:30: sparse:    expected void const [noderef] __percpu *__vpp_verify
  cacheinfo.c:282:30: sparse:    got unsigned int *

The reason is that on UP builds cpu_info which is a per CPU variable on SMP
is mapped to boot_cpu_info which is a regular variable. There is a hideous
accessor cpu_data() which tries to hide this, but it's not sufficient as
some places require raw accessors and generates worse code than the regular
per CPU accessors.

Waste sizeof(struct x86_cpuinfo) memory on UP and provide the per CPU
cpu_info unconditionally. This requires to update the CPU info on the boot
CPU as SMP does. (Ab)use the weakly defined smp_prepare_boot_cpu() function
and implement exactly that.

This allows to use regular per CPU accessors uncoditionally and paves the
way to remove the cpu_data() hackery.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240304005104.622511517@linutronix.de

arch/x86/include/asm/processor.h

@@ -185,13 +185,8 @@ extern struct cpuinfo_x86	new_cpu_data;
 extern __u32			cpu_caps_cleared[NCAPINTS + NBUGINTS];
 extern __u32			cpu_caps_set[NCAPINTS + NBUGINTS];
 
-#ifdef CONFIG_SMP
 DECLARE_PER_CPU_READ_MOSTLY(struct cpuinfo_x86, cpu_info);
 #define cpu_data(cpu)		per_cpu(cpu_info, cpu)
-#else
-#define cpu_info		boot_cpu_data
-#define cpu_data(cpu)		boot_cpu_data
-#endif
 
 extern const struct seq_operations cpuinfo_op;
 

arch/x86/kernel/cpu/common.c

@@ -70,6 +70,9 @@
 
 #include "cpu.h"
 
+DEFINE_PER_CPU_READ_MOSTLY(struct cpuinfo_x86, cpu_info);
+EXPORT_PER_CPU_SYMBOL(cpu_info);
+
 u32 elf_hwcap2 __read_mostly;
 
 /* Number of siblings per CPU package */

arch/x86/kernel/setup.c

@@ -1211,6 +1211,16 @@ 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
 };

arch/x86/kernel/smpboot.c

@@ -101,10 +101,6 @@ EXPORT_PER_CPU_SYMBOL(cpu_core_map);
 DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_die_map);
 EXPORT_PER_CPU_SYMBOL(cpu_die_map);
 
-/* Per CPU bogomips and other parameters */
-DEFINE_PER_CPU_READ_MOSTLY(struct cpuinfo_x86, cpu_info);
-EXPORT_PER_CPU_SYMBOL(cpu_info);
-
 /* CPUs which are the primary SMT threads */
 struct cpumask __cpu_primary_thread_mask __read_mostly;