本文介绍Linux kernel是如何读取CPU支持哪些cpu features,并是如何将CPUID指令查询的cpu features保存到boot_cpu_data.x86_capabilities结构中的。
#CPUID在kernel中的组织
在kernel启动的时候，会读取平台支持的CPUID features，并存储在boot_cpu_data.x86_capability[]数组中。
在kernel实际运行的时候，直接查询boot_cpu_data.x86_capability[]数组，而不是直接运行CPUID 指令。
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in init/main.c1
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29asmlinkage __visible void __init start_kernel(void)
{
...
boot_cpu_init();
...
setup_arch(&command_line);
...
}
void __init setup_arch(char **cmdline_p)
{
...
memcpy(&boot_cpu_data, &new_cpu_data, sizeof(new_cpu_data));
...
printk(KERN_INFO "Command line: %s\n", boot_command_line);
boot_cpu_data.x86_phys_bits = MAX_PHYSMEM_BITS;
...
early_cpu_init();
...
}
void __init early_cpu_init(void)
{
...
early_identify_cpu(&boot_cpu_data);
}
可以看到从start_kernel()开始，会调用setup_arch()，然后调用early_cpu_init()函数，在early_cpu_init()函数中调用early_identify_cpu(&boot_cpu_data)函数来初始化boot_cpu_data数据结构。下面我们来看early_identify_cpu()函数。1
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70static void __init early_identify_cpu(struct cpuinfo_x86 *c)
{
c->x86_clflush_size = 64;
c->x86_phys_bits = 36;
c->x86_virt_bits = 48;
c->x86_clflush_size = 32;
c->x86_phys_bits = 32;
c->x86_virt_bits = 32;
c->x86_cache_alignment = c->x86_clflush_size;
memset(&c->x86_capability, 0, sizeof(c->x86_capability));
c->extended_cpuid_level = 0;
if (!have_cpuid_p())
identify_cpu_without_cpuid(c);
/* cyrix could have cpuid enabled via c_identify()*/
if (have_cpuid_p()) {
cpu_detect(c);
get_cpu_vendor(c);
get_cpu_cap(c);
get_cpu_address_sizes(c);
setup_force_cpu_cap(X86_FEATURE_CPUID);
if (this_cpu->c_early_init)
this_cpu->c_early_init(c);
c->cpu_index = 0;
filter_cpuid_features(c, false);
if (this_cpu->c_bsp_init)
this_cpu->c_bsp_init(c);
} else {
setup_clear_cpu_cap(X86_FEATURE_CPUID);
}
setup_force_cpu_cap(X86_FEATURE_ALWAYS);
cpu_set_bug_bits(c);
fpu__init_system(c);
/*
* Regardless of whether PCID is enumerated, the SDM says
* that it can't be enabled in 32-bit mode.
*/
setup_clear_cpu_cap(X86_FEATURE_PCID);
/*
* Later in the boot process pgtable_l5_enabled() relies on
* cpu_feature_enabled(X86_FEATURE_LA57). If 5-level paging is not
* enabled by this point we need to clear the feature bit to avoid
* false-positives at the later stage.
*
* pgtable_l5_enabled() can be false here for several reasons:
* - 5-level paging is disabled compile-time;
* - it's 32-bit kernel;
* - machine doesn't support 5-level paging;
* - user specified 'no5lvl' in kernel command line.
*/
if (!pgtable_l5_enabled())
setup_clear_cpu_cap(X86_FEATURE_LA57);
detect_nopl();
}
early_identify_cpu()函数会先将boot_cpu_data.x86_capability清零；设置c->extended_cpuid_level = 0;
然后判断CPU是否支持CPUID指令（if(have_cpuid_p()），支持则运行if语句：
- cpu_detect()函数，设置boot_cpu_data的cpuid_level, x86_vendor_id[0], x86_vendor_id[8], x86_vendor_id[4], x86, x86_model, x86_stepping, x86_clflush_size, x86_cache_alignment;
- get_cpu_vendor()
- get_cpu_cap(c)
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96void get_cpu_cap(struct cpuinfo_x86 *c)
{
u32 eax, ebx, ecx, edx;
/* Intel-defined flags: level 0x00000001 */
if (c->cpuid_level >= 0x00000001) {
cpuid(0x00000001, &eax, &ebx, &ecx, &edx);
c->x86_capability[CPUID_1_ECX] = ecx;
c->x86_capability[CPUID_1_EDX] = edx;
}
/* Thermal and Power Management Leaf: level 0x00000006 (eax) */
if (c->cpuid_level >= 0x00000006)
c->x86_capability[CPUID_6_EAX] = cpuid_eax(0x00000006);
/* Additional Intel-defined flags: level 0x00000007 */
if (c->cpuid_level >= 0x00000007) {
cpuid_count(0x00000007, 0, &eax, &ebx, &ecx, &edx);
c->x86_capability[CPUID_7_0_EBX] = ebx;
c->x86_capability[CPUID_7_ECX] = ecx;
c->x86_capability[CPUID_7_EDX] = edx;
}
/* Extended state features: level 0x0000000d */
if (c->cpuid_level >= 0x0000000d) {
cpuid_count(0x0000000d, 1, &eax, &ebx, &ecx, &edx);
c->x86_capability[CPUID_D_1_EAX] = eax;
}
/* Additional Intel-defined flags: level 0x0000000F */
if (c->cpuid_level >= 0x0000000F) {
/* QoS sub-leaf, EAX=0Fh, ECX=0 */
cpuid_count(0x0000000F, 0, &eax, &ebx, &ecx, &edx);
c->x86_capability[CPUID_F_0_EDX] = edx;
if (cpu_has(c, X86_FEATURE_CQM_LLC)) {
/* will be overridden if occupancy monitoring exists */
c->x86_cache_max_rmid = ebx;
/* QoS sub-leaf, EAX=0Fh, ECX=1 */
cpuid_count(0x0000000F, 1, &eax, &ebx, &ecx, &edx);
c->x86_capability[CPUID_F_1_EDX] = edx;
if ((cpu_has(c, X86_FEATURE_CQM_OCCUP_LLC)) ||
((cpu_has(c, X86_FEATURE_CQM_MBM_TOTAL)) ||
(cpu_has(c, X86_FEATURE_CQM_MBM_LOCAL)))) {
c->x86_cache_max_rmid = ecx;
c->x86_cache_occ_scale = ebx;
}
} else {
c->x86_cache_max_rmid = -1;
c->x86_cache_occ_scale = -1;
}
}
/* AMD-defined flags: level 0x80000001 */
eax = cpuid_eax(0x80000000);
c->extended_cpuid_level = eax;
if ((eax & 0xffff0000) == 0x80000000) {
if (eax >= 0x80000001) {
cpuid(0x80000001, &eax, &ebx, &ecx, &edx);
c->x86_capability[CPUID_8000_0001_ECX] = ecx;
c->x86_capability[CPUID_8000_0001_EDX] = edx;
}
}
if (c->extended_cpuid_level >= 0x80000007) {
cpuid(0x80000007, &eax, &ebx, &ecx, &edx);
c->x86_capability[CPUID_8000_0007_EBX] = ebx;
c->x86_power = edx;
}
if (c->extended_cpuid_level >= 0x80000008) {
cpuid(0x80000008, &eax, &ebx, &ecx, &edx);
c->x86_capability[CPUID_8000_0008_EBX] = ebx;
}
if (c->extended_cpuid_level >= 0x8000000a)
c->x86_capability[CPUID_8000_000A_EDX] = cpuid_edx(0x8000000a);
init_scattered_cpuid_features(c);
init_speculation_control(c);
/*
* Clear/Set all flags overridden by options, after probe.
* This needs to happen each time we re-probe, which may happen
* several times during CPU initialization.
*/
apply_forced_caps(c);
}
get_cpu_cap()函数会
