Re: Page table and memory management

[Richard <judicator3@gmail.com>]

Hi Keir,

I am still having trouble to edit the page table entries with the
mmu_update hypercall.
I am actually updating the mini-os. The mem management in mini-os is
pretty outdated. For example, it assumes that the initial page tables
are placed at the top of available memory and hence does not count
properly the total amount of pages that have been allocated to the
domain. I am extending the initial PGD and PT given to mini-os at boot
time to map the available memory that I can use.

I tried to first pin a level 1 page frame before inserting it as an
entry in the level 2 PGD.
Whether I pin a level1 page frame first or I start using it directly
as a page table, it does not matter the mmu_update hypercall still
fails.

I traced through the XEN code, in the file arch/x86/memory.c. Whether
excuting a MMU_NORMAL_PT_UPDATE or a MMU_EXTENDED_COMMAND type of
command, the function do_mmu_update() eventually calls the helper
function get_page_and_type_from_pagenr() which in turns call the
function get_page_type(). Below I drew a call graph.

                                                 do_mmu_update
                                                            |
                                                            |
                      
/------------------------------------------------------------------------------\
                       |                                              
                               |
                       \/                                             
                               \/
               mod_l2_entry                                           
   do_extended_command
                       |                                              
                               |
                       \/                                             
                               |
            get_page_from_l2e                                         
                    |
                       |                                              
                               |
                       |                                              
                               |
                      
\------------------------------------------------------------------------------/
                                                              |
                                                              \/
                                        get_page_and_type_from_pagenr
                                                              |
                                                              \/
                                                     get_page_type

The get_page_type() checks the upperbits of the 'typeinfo' field of
the page frame to make sure it is of type 'PGT_l1_page_table'.
get_page_type() also does another check to verify that the 'typeinfo'
field also contains the 10 upper bits of the virtual address that this
page frame is suppose to eventually map to.

> Xen will automatically infer the type when you attach an L1 to an
> existing L2. Xen will infer the L2 type when the L2 gets used as
> current pagetable base.
So I do not see where XEN is automatically inferring the L1 type when
I am inserting for the 1st time the L1 page frame into the L2. In
order to use a page frame as an L1 page table, I have to find a way to
tell XEN to update the corresponding typeinfo field in order to pass
the verification in get_page_type().

Thanks
Richard


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