How does the Linux kernel get info about the processors and the cores?

Question

Assume we have a blank computer without any OS and we are installing a Linux. Where in the kernel is the code that identifies the processors and the cores and get information about/from them? This info eventually shows up in places like /proc/cpuinfo but how does the kernel get it in the first place?!

Answer 1

Short answer

Kernel uses special CPU instruction cpuid and saves results in internal structure - cpuinfo_x86 for x86

Long answer

Kernel source is your best friend. Start from entry point - file /proc/cpuinfo. As any proc file it has to be cretaed somewhere in kernel and declared with some file_operations. This is done at fs/proc/cpuinfo.c. Interesting piece is seq_open that uses reference to some cpuinfo_op. This ops are declared in arch/x86/kernel/cpu/proc.c where we see some show_cpuinfo function. This function is in the same file on line 57.

Here you can see

 64         seq_printf(m, "processor\t: %u\n"
 65                    "vendor_id\t: %s\n"
 66                    "cpu family\t: %d\n"
 67                    "model\t\t: %u\n"
 68                    "model name\t: %s\n",
 69                    cpu,
 70                    c->x86_vendor_id[0] ? c->x86_vendor_id : "unknown",
 71                    c->x86,
 72                    c->x86_model,
 73                    c->x86_model_id[0] ? c->x86_model_id : "unknown");

Structure c declared on the first line as struct cpuinfo_x86. This structure is declared in arch/x86/include/asm/processor.h. And if you search for references on that structure you will find function cpu_detect and that function calls function cpuid which is finally resolved to native_cpuid that looks like this:

189 static inline void native_cpuid(unsigned int *eax, unsigned int *ebx,
190                                 unsigned int *ecx, unsigned int *edx)
191 {
192         /* ecx is often an input as well as an output. */
193         asm volatile("cpuid"
194             : "=a" (*eax),
195               "=b" (*ebx),
196               "=c" (*ecx),
197               "=d" (*edx)
198             : "" (*eax), "2" (*ecx)
199             : "memory");
200 }

And here you see assembler instruction cpuid. And this little thing does real work.

Answer 2

This information from BIOS + Hardware DB. You can get info direct by dmidecode, for example (if you need more info - try to check dmidecode source code)

sudo dmidecode -t processor