[Xen-devel] idle_pg_tables??

[PUCCETTI Armand <armand.puccetti@xxxxxx>]

In the paging mechanism of XEN what is the role of the variable 
'idle_pg_table*' variables ??
For a 4-levels paging system these variables are defined in x86_64.S and 
partially initialised.
Here is the code, copied from x86_64.S:

__________________________________________________
...

/* Initial PML4 -- level-4 page table. */
       .org 0x2000
ENTRY(idle_pg_table)
ENTRY(idle_pg_table_4)
       .quad idle_pg_table_l3 - __PAGE_OFFSET + 7 # PML4[0]
       .fill 261,8,0
       .quad idle_pg_table_l3 - __PAGE_OFFSET + 7 # PML4[262]

/* Initial PDP -- level-3 page table. */
       .org 0x3000
ENTRY(idle_pg_table_l3)
       .quad idle_pg_table_l2 - __PAGE_OFFSET + 7

/* Initial PDE -- level-2 page table. Maps first 64MB physical memory. */
       .org 0x4000
ENTRY(idle_pg_table_l2)
       .macro identmap from=0, count=32
       .if \count-1
       identmap "(\from+0)","(\count/2)"
       identmap "(\from+(0x200000*(\count/2)))","(\count/2)"
       .else
       .quad 0x00000000000001e3 + \from
       .endif
       .endm
       identmap

       .org 0x4000 + PAGE_SIZE
       .code64

.section ".bss.stack_aligned","w"
ENTRY(cpu0_stack)
       .fill STACK_SIZE,1,0
______________________________________________________
trying to understand that:

- idle_pg_table_l4 is the same as idle_pg_table and contains 263 enties, 
all zeroed but two (identical) ones. These
two pointers point somewhere close to idle_pg_table_l3. Why are there 
two identical pointers and why shift them by __PAGE_OFFSET +7?
- idle_pg_table_l3 is located between 0x3000 and 0x4000 , with only the 
first slot initialised. The later points to
level 2 table with some offset.

- idle_pg_table_l2 has terrible code with a recursive macro, who expands 
into 63 quad constants. It is unclear
to me why this complicated macro?? I would have put a table of constants 
pretty simply... Every entry in that l2 table points to a
fixed address, at intervals of 4K (a page).l2 tables are located between 
0x01E3 to 0x03E001E3 in groups. Every group
is apparently a set of 4 page tables and each table has a size of 128K. 
Groups are separated by approx 256MB.
Why are these spacings and groups?

- idle_pg_table_l1 is not an entry and so l1 tables are not allocated. Why?

thanks for help!

Armand


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