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SubjectRe: [PATCH 6/6] x86/vdso: Move out the CPU number store
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On 06/05/2018 07:44 AM, Bae, Chang Seok wrote:
>>> diff --git a/arch/x86/kernel/setup_percpu.c b/arch/x86/kernel/setup_percpu.c
>>> index ea554f8..e716e94 100644
>>> --- a/arch/x86/kernel/setup_percpu.c
>>> +++ b/arch/x86/kernel/setup_percpu.c
>>> @@ -155,12 +155,21 @@ static void __init pcpup_populate_pte(unsigned long addr)
>>>
>>> static inline void setup_percpu_segment(int cpu)
>>> {
>>> -#ifdef CONFIG_X86_32
>>> - struct desc_struct d = GDT_ENTRY_INIT(0x8092, per_cpu_offset(cpu),
>>> - 0xFFFFF);
>>> +#ifdef CONFIG_NUMA
>>> + unsigned long node = early_cpu_to_node(cpu);
>>> +#else
>>> + unsigned long node = 0;
>>> +#endif
>>> + struct desc_struct d = GDT_ENTRY_INIT(0x0, per_cpu_offset(cpu),
>>> + make_lsl_tscp(cpu, node));
>>> +
>>> + d.type = 5; /* R0 data, expand down, accessed */
>>> + d.dpl = 3; /* Visible to user code */
>>> + d.s = 1; /* Not a system segment */
>>> + d.p = 1; /* Present */
>>> + d.d = 1; /* 32-bit */
>>>
>>> write_gdt_entry(get_cpu_gdt_rw(cpu), GDT_ENTRY_PERCPU, &d, DESCTYPE_S);
>>> -#endif
>>> }
>
>> This won't work on X86-32 because it actually uses the segment limit with fs: access. So there
>> is a reason why the lsl based method is X86-64 only.
>
> The limit will be consumed in X86-64 only, while the unification with i386 was suggested for a
> different reason.
>
> Thanks,
> Chang
>

The unification affects i386, and the limit is consumed by the processor with fs: access.
The limit was 0xFFFFF before, now it depends on the cpu/node. So accesses on small number cpus
are likely to fault.

--Mika

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