linux-kernel - Re: [x86_64] Question about early page tables initialization

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Date:	Tue, 03 Feb 2015 14:42:28 +0200
From:	Oren Twaig <oren@...lemp.com>
To:	Alex Kuleshov <kuleshovmail@...il.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [x86_64] Question about early page tables initialization

Hi,

This is the corresponding C code which can help you understand:

   u64 *pml4 = (u64*)pgtable;
   u64 pdp = pgtable + 0x1000;
   u64 pml4_entry = pdp | PTE_P | PTE_W | PTU; // present, write, userspace = 0x7
   pml4[0] = pml4_entry;

The 0x1007 you see is just the calculation of the pml4_entry.

Oren Twaig.

On 02/03/2015 02:25 PM, Alex Kuleshov wrote:
> Hello All,
>
> I have a question about page tables initialization in the
> arch/x86/boot/compressed/head_64.S
>
> After we clear memory for page tables, there is code which
> build PML4:
>
>     leal    pgtable + 0(%ebx), %edi
>     leal    0x1007(%edi), %eax     
>     movl    %eax, 0(%edi)
>        
> Why there is offset 0x1007 instead just 0x7? 0x1007 is
> 4k + 7bit (PML4E) flags as i understand correctly. But
> why we skip first 4k here?
>
> Thank you.
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