x86: Fix boot failures on older AMD CPU's

People with old AMD chips are getting hung boots, because commit
bcb80e53877c ("x86, microcode, AMD: Add microcode revision to
/proc/cpuinfo") moved the microcode detection too early into
"early_init_amd()".

At that point we are *so* early in the booth that the exception tables
haven't even been set up yet, so the whole

rdmsr_safe(MSR_AMD64_PATCH_LEVEL, &c->microcode, &dummy);

doesn't actually work: if the rdmsr does a GP fault (due to non-existant
MSR register on older CPU's), we can't fix it up yet, and the boot fails.

Fix it by simply moving the code to a slightly later point in the boot
(init_amd() instead of early_init_amd()), since the kernel itself
doesn't even really care about the microcode patchlevel at this point
(or really ever: it's made available to user space in /proc/cpuinfo, and
updated if you do a microcode load).

Reported-tested-and-bisected-by: Larry Finger <[e-mail]>
Tested-by: Bob Tracy <[e-mail]>
Acked-by: Borislav Petkov <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Srivatsa S. Bhat <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/x86/kernel/cpu/amd.c

···
 
 static void __cpuinit early_init_amd(struct cpuinfo_x86 *c)
 {
-	u32 dummy;
-
 	early_init_amd_mc(c);
 
 	/*
···
 			set_cpu_cap(c, X86_FEATURE_EXTD_APICID);
 	}
 #endif
-
-	rdmsr_safe(MSR_AMD64_PATCH_LEVEL, &c->microcode, &dummy);
 }
 
 static void __cpuinit init_amd(struct cpuinfo_x86 *c)
 {
+	u32 dummy;
+
 #ifdef CONFIG_SMP
 	unsigned long long value;
 
···
 			checking_wrmsrl(MSR_AMD64_MCx_MASK(4), mask);
 		}
 	}
+
+	rdmsr_safe(MSR_AMD64_PATCH_LEVEL, &c->microcode, &dummy);
 }
 
 #ifdef CONFIG_X86_32