linux-kernel - Re: [PATCH 6/6] x86/vdso: Move out the CPU number store

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Message-ID: <C62C3F95-0DD4-4B66-89A8-4FAB819C8A70@intel.com>
Date:   Tue, 5 Jun 2018 04:44:16 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     Mika Penttilä <mika.penttila@...tfour.com>,
        "Andy Lutomirski" <luto@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
CC:     Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Metzger, Markus T" <markus.t.metzger@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/6] x86/vdso: Move out the CPU number store

>> 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