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SubjectAPIC logic bug in kernel
Greetings list.

I'm looking at the code in arch/x86/kernel/apic/apic.c specifically the
code path that checks for x86_vendor == X86_VENDOR_INTEL with
boot_cpu_data.x86 = 5.

As I understand the code, BIOS will have informed kernel that it has an
APIC based on mps tables. If the CPU family == 5, the function
apic_verify() will be called.

Problem is apic_verify() does an rdmsr for an MSR that was not included
in IA until P6.

Specifically rdmsr/wrmsr instructions in apic.c are not P5 compatible -
since the MSR IA32_APIC_BASE was not introduced until P6_01 - as listed
in the system programming guide volume 3.

Are all of these rdmsr/wrmsr calls made with an awareness of P5 ?

Example:

We check in apic_verify() if cpuid(1).edx has bit 9 (local APIC) set.
If so we set CPU capability FEATURE_APIC and rdmsr/wrmsr to 0x1B

arch/x86/include/asm/msr-index.h:#define MSR_IA32_APICBASE 0x0000001b
As I read this code it is perfectly valid for a P5, to have an APIC,
report it has APIC capability via BIOS and CPUID and then subsequently
to go ahead and touch the IA32_APIC_BASE MSR.

Basically this code doesn't seem to match the spec, am I missing a trick ?


Bryan


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