Re: [PATCH 2.4.24] Fix GDT limit in setup.S

[Coywolf Qi Hunt]

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.


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

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