x86/cpu: Add helper function to get the type of the current hybrid CPU

On processors with Intel Hybrid Technology (i.e., one having more than
one type of CPU in the same package), all CPUs support the same
instruction set and enumerate the same features on CPUID. Thus, all
software can run on any CPU without restrictions. However, there may be
model-specific differences among types of CPUs. For instance, each type
of CPU may support a different number of performance counters. Also,
machine check error banks may be wired differently. Even though most
software will not care about these differences, kernel subsystems
dealing with these differences must know.

Add and expose a new helper function get_this_hybrid_cpu_type() to query
the type of the current hybrid CPU. The function will be used later in
the perf subsystem.

The Intel Software Developer's Manual defines the CPU type as 8-bit
identifier.

Signed-off-by: Ricardo Neri <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Reviewed-by: Tony Luck <[e-mail]>
Reviewed-by: Len Brown <[e-mail]>
Acked-by: Borislav Petkov <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]

arch/x86/include/asm/cpu.h

···
 extern void switch_to_sld(unsigned long tifn);
 extern bool handle_user_split_lock(struct pt_regs *regs, long error_code);
 extern bool handle_guest_split_lock(unsigned long ip);
+u8 get_this_hybrid_cpu_type(void);
 #else
 static inline void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c) {}
 static inline void switch_to_sld(unsigned long tifn) {}
···
 static inline bool handle_guest_split_lock(unsigned long ip)
 {
 	return false;
+}
+
+static inline u8 get_this_hybrid_cpu_type(void)
+{
+	return 0;
 }
 #endif
 #ifdef CONFIG_IA32_FEAT_CTL

arch/x86/kernel/cpu/intel.c

···
 	cpu_model_supports_sld = true;
 	split_lock_setup();
 }
+
+#define X86_HYBRID_CPU_TYPE_ID_SHIFT	24
+
+/**
+ * get_this_hybrid_cpu_type() - Get the type of this hybrid CPU
+ *
+ * Returns the CPU type [31:24] (i.e., Atom or Core) of a CPU in
+ * a hybrid processor. If the processor is not hybrid, returns 0.
+ */
+u8 get_this_hybrid_cpu_type(void)
+{
+	if (!cpu_feature_enabled(X86_FEATURE_HYBRID_CPU))
+		return 0;
+
+	return cpuid_eax(0x0000001a) >> X86_HYBRID_CPU_TYPE_ID_SHIFT;
+}