[PATCH for 4.19?] x86/Intel: unlock CPUID earlier for the BSP

[Jan Beulich <jbeulich@xxxxxxxx>]

Intel CPUs have a MSR bit to limit CPUID enumeration to leaf two. If
this bit is set by the BIOS then CPUID evaluation does not work when
data from any leaf greater than two is needed; early_cpu_init() in
particular wants to collect leaf 7 data.

Cure this by unlocking CPUID right before evaluating anything which
depends on the maximum CPUID leaf being greater than two.

Inspired by (and description cloned from) Linux commit 0c2f6d04619e
("x86/topology/intel: Unlock CPUID before evaluating anything").

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
While I couldn't spot anything, it kind of feels as if I'm overlooking
further places where we might be inspecting in particular leaf 7 yet
earlier.

No Fixes: tag(s), as imo it would be too many that would want
enumerating.

--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -336,7 +336,8 @@ void __init early_cpu_init(bool verbose)
 
        c->x86_vendor = x86_cpuid_lookup_vendor(ebx, ecx, edx);
        switch (c->x86_vendor) {
-       case X86_VENDOR_INTEL:    actual_cpu = intel_cpu_dev;    break;
+       case X86_VENDOR_INTEL:    intel_unlock_cpuid_leaves(c);
+                                 actual_cpu = intel_cpu_dev;    break;
        case X86_VENDOR_AMD:      actual_cpu = amd_cpu_dev;      break;
        case X86_VENDOR_CENTAUR:  actual_cpu = centaur_cpu_dev;  break;
        case X86_VENDOR_SHANGHAI: actual_cpu = shanghai_cpu_dev; break;
--- a/xen/arch/x86/cpu/cpu.h
+++ b/xen/arch/x86/cpu/cpu.h
@@ -24,3 +24,5 @@ void amd_init_lfence(struct cpuinfo_x86
 void amd_init_ssbd(const struct cpuinfo_x86 *c);
 void amd_init_spectral_chicken(void);
 void detect_zen2_null_seg_behaviour(void);
+
+void intel_unlock_cpuid_leaves(struct cpuinfo_x86 *c);
--- a/xen/arch/x86/cpu/intel.c
+++ b/xen/arch/x86/cpu/intel.c
@@ -303,10 +303,24 @@ static void __init noinline intel_init_l
                ctxt_switch_masking = intel_ctxt_switch_masking;
 }
 
-static void cf_check early_init_intel(struct cpuinfo_x86 *c)
+/* Unmask CPUID levels if masked. */
+void intel_unlock_cpuid_leaves(struct cpuinfo_x86 *c)
 {
-       u64 misc_enable, disable;
+       uint64_t misc_enable, disable;
+
+       rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
+
+       disable = misc_enable & MSR_IA32_MISC_ENABLE_LIMIT_CPUID;
+       if (disable) {
+               wrmsrl(MSR_IA32_MISC_ENABLE, misc_enable & ~disable);
+               bootsym(trampoline_misc_enable_off) |= disable;
+               c->cpuid_level = cpuid_eax(0);
+               printk(KERN_INFO "revised cpuid level: %u\n", c->cpuid_level);
+       }
+}
 
+static void cf_check early_init_intel(struct cpuinfo_x86 *c)
+{
        /* Netburst reports 64 bytes clflush size, but does IO in 128 bytes */
        if (c->x86 == 15 && c->x86_cache_alignment == 64)
                c->x86_cache_alignment = 128;
@@ -315,16 +329,7 @@ static void cf_check early_init_intel(st
            bootsym(trampoline_misc_enable_off) & 
MSR_IA32_MISC_ENABLE_XD_DISABLE)
                printk(KERN_INFO "re-enabled NX (Execute Disable) 
protection\n");
 
-       /* Unmask CPUID levels and NX if masked: */
-       rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
-
-       disable = misc_enable & MSR_IA32_MISC_ENABLE_LIMIT_CPUID;
-       if (disable) {
-               wrmsrl(MSR_IA32_MISC_ENABLE, misc_enable & ~disable);
-               bootsym(trampoline_misc_enable_off) |= disable;
-               printk(KERN_INFO "revised cpuid level: %d\n",
-                      cpuid_eax(0));
-       }
+       intel_unlock_cpuid_leaves(c);
 
        /* CPUID workaround for Intel 0F33/0F34 CPU */
        if (boot_cpu_data.x86 == 0xF && boot_cpu_data.x86_model == 3 &&