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SubjectRe: A problem with percpu variable cpu_number
From
Hillf Danton:

Thanks for your remind
and sorry for my poor English.


2012/2/22 Hillf Danton <dhillf@gmail.com>:
> On Wed, Feb 22, 2012 at 7:13 PM, Tao Jiang <jiangtao.jit@gmail.com> wrote:
>> Hi:
>>
>> Thank you all.
>>
>> So in boot_cpu_init(), it will always set bit 0 to these masks.
>> If the boot cpu is the first processor, it's the right case.
>> And if the BP is not the first one, is it wrong?
>> But can it happen that the BP is not cpu0?
>
> BP could be not "the first one".
> Due to that current code boots fine, I guess, you mess
> logic CPU with hard CPU.
>
> btw, replying messages in this way looks not fine.
>
>> Thank you.
>>
>>
>> 2012/2/22 Brian Gerst <brgerst@gmail.com>:
>>> On Tue, Feb 21, 2012 at 9:29 AM, Cong Wang <xiyou.wangcong@gmail.com> wrote:
>>>> On 02/21/2012 08:41 PM, Tao Jiang wrote:
>>>>>
>>>>> Hi Cong Wang:
>>>>>
>>>>> I read the file vmlinux.lds.S in arch/x86/kernel
>>>>> section .data..percpu is between .init.data and .init.end
>>>>> Is that means these percpu variables will be freed after init?
>>>>
>>>>
>>>> % grep -e __init_begin -e __init_end -e __per_cpu_start -e __per_cpu_end
>>>> /boot/System.map
>>>> 0000000000000000 D __per_cpu_start
>>>> 0000000000014bc0 D __per_cpu_end
>>>> ffffffff81cf3000 D __init_begin
>>>> ffffffff81dfc000 R __init_end
>>>> % objdump -d -j .data..percpu vmlinux | grep cpu_number
>>>> 000000000000dc38 <cpu_number>:
>>>
>>> The .data..percpu section is placed in the init section, but x86-64 is
>>> a special case as noted below.  The boot cpu is pointed to the init
>>> percpu section until setup_per_cpu_areas() is called, when it switches
>>> to the regular percpu area.  The init percpu data is then freed with
>>> all other init data.
>>>
>>> The reason the percpu symbols start at virtual address 0 on x86-64 is
>>> because of the requirement that gs_base must be a canonical address
>>> (it cannot be a simple offset like x86-32).  But, the data is still
>>> loaded in the init section in memory.  See
>>> arch/x86/kernel/vmlinux.lds.S for the explanation of how the linker
>>> changes the program headers to set the virtual address to zero but
>>> keeps the load address in the init section.
>>>
>>> To answer the original question. in the case of cpu_number, it is set
>>> to zero in the init section because it doesn't have an explicit
>>> initializer.  Therefore the boot cpu will always read zero for the
>>> cpu_number, even before setup_per_cpu_areas() is called.
>>>
>>>
>>> --
>>> Brian Gerst
>> --
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