Re: [Xen-devel] understanding __linear_l2_table and friends

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

They aren't actually used during domain building, but anyway: Xen uses
the common trick whereby each page directory maps itself. This means
that every page-table entry is mapped into the address space at some
virtual address. In fact, page directory entries (and PML3 and PML4
entries on x86/64) are also directly accessible in the virtual address
space. The macros below are expressions that evaluate to the correct
virtual addresses.

 -- Keir

> I was trying to understand the states behind domain creation, but i'm
> having troubles getting past this. Would someone mind saying a few
> words about what these are and (if still needed) why these calculations
> work for that?
> 
> xen/include/asm-x86/page.h:
>    #define linear_l1_table                                                 \
>        ((l1_pgentry_t *)(LINEAR_PT_VIRT_START))
>    #define __linear_l2_table                                                 \
>        ((l2_pgentry_t *)(LINEAR_PT_VIRT_START +                            \
>                         (LINEAR_PT_VIRT_START >> (PAGETABLE_ORDER<<0))))
>    #define __linear_l3_table                                                 \
>        ((l3_pgentry_t *)(LINEAR_PT_VIRT_START +                            \
>                         (LINEAR_PT_VIRT_START >> (PAGETABLE_ORDER<<0)) +   \
>                         (LINEAR_PT_VIRT_START >> (PAGETABLE_ORDER<<1))))
>    #define __linear_l4_table                                                 \
>        ((l4_pgentry_t *)(LINEAR_PT_VIRT_START +                            \
>                         (LINEAR_PT_VIRT_START >> (PAGETABLE_ORDER<<0)) +   \
>                         (LINEAR_PT_VIRT_START >> (PAGETABLE_ORDER<<1)) +   \
>                         (LINEAR_PT_VIRT_START >> (PAGETABLE_ORDER<<2))))
>    
> Thanks!
> sRp
> 
> -- 
> Scott Parish
> 
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> http://lists.xensource.com/xen-devel
> 


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