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DateThu, 19 Feb 2004 16:33:11 +0800
FromCoywolf Qi Hunt <>
Subject{ Linux not only practical, but an ideal. } Re: [PATCH 2.4.24] Fix GDT limit in setup.S
Coywolf Qi Hunt wrote:

> Richard B. Johnson wrote:
>
>> On Tue, 17 Feb 2004, Coywolf Qi Hunt wrote:
>>
>>
>>> Hello 2.4.xx hackers,
>>>
>>> In setup.S, i feel like that the gdt limit 0x8000 is not proper and it
>>> should be 0x800. How came 0x800 into 0x8000 in 2.4.xx code? Is there a
>>> story? It shouldn't be a careless typo. 256 gdt entries should be
>>> enough and since it's boot gdt, 256 is ok even if the code is run on SMP
>>> with 64 cpus.
>>>
>>
>>
>> The first element has nothing to do with the number of GDT entries.
>> It represents the LIMIT. Because the granularity bit is
>> set meaning 4 kilobyte pages and 0x8000 * 0x1000 = 0x8000000
>> | |
>> | |_______ Page size
>> |______________ GDT value
>> This is the size of address space that is unity-mapped for boot.
>>
>> The granularity is also not the number of GDT entries. It
>> represents the length for which the GDT definition applies.
>>
>> Because this GDT is used only for booting, somebody decided that
>> there would never be any boot code beyond 2 GB so there was no
>> reason to make room for it. If you change the number to 0x0800,
>> you are declaring that neither the boot code nor any RAM-disk
>> combination will ever exceed 0x800 * 0x1000 = 0x800000 bytes.
>> Therefore you broke my imbedded system. Do not do this.
>>
>>
>>> At least the comment doesn't match the code. Either fix the code or fix
>>> the comment. We really needn't so many GDT entries. Let's use the intel
>>> segmentation in a most limited way. Below follows a patch fixing the
>>> code.
>>>
>>> I don't have the latest 2.4.24, but setup.S isn't changed from 2.4.23 to
>>> 2.4.24.
>>>
>>> Regards, Coywolf
>>>
>
> Thank you for you kindly reply, though I really don not agree with you.
> But thanks very much, since you are the only one replying me. I CCed to
> all the 2.0 2.2 2.4 maintainers.
>
>

Hello Richard B. Johnson,

I've just find out where the problem lying in your answer, that you are
not talking the same thing as i am. You are talking about the descriptor
on which you are quite correct, while I am talking about GDTR, similar
to selector, but not selector, could be called `selector' in GDTR. Two
different things.

To All Hackers,

plz take my patch to make our kernel more perfect. Linux should be not
only practical, but an ideal. And i have more such patches to send.


Regards,
Coywolf


--
Coywolf Qi Hunt
Admin of http://GreatCN.org and http://LoveCN.org

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