Page table and memory management

Page table and memory management

[Richard]

Hello,

I am having some problems to update the page table entries of my
domain with the hypervisor call do_mmu_update().

With 2 level paging XEN refers to the page global directory (PGD) as
level2 and page tables (PT) as level1. I am taking a page frame that
my domain owns, I am going to use that page frame as a page table
(level1 PT) and I am adding the 'address' of that page frame as an
entry of my page global directory (level2 PGD). So my request
'mmu_update_t' to the hypercall do_mmu_update() looks like this.

mmu_update_t.ptr = machine address of PGD entry that I want to update
mmu_update_t.val = machine frame number of PT with appropriate flags

do_mmu_update() and its helper functions in xen/arch/x86/memory.c do
many checks. One of the checks is to verify the flags in '(struct
pfn_info).u.inuse.type_info'. The upper bits of 'type_info' contain
the type of the page frame: PGT_none, PGT_l1_page_table,
PGT_l2_page_table etc ...

In my case, do_mmu_update() verifies that the page frame that I am
going to use as a page table is actually of type PGT_l1_page_table.
Therefore, I cannot just take any random page frame that my domain
owns and use it as page table.

How do I create a new page table ? 
How do I register a page frame that I own with XEN so that I can use
it as a page table ?

I am trying to see how XenoLinux is doing its page table allocation.
>From what I can see, function pte_alloc_map() in mm/memory.c is not
doing anything special. It is just getting a free page frame and using
the function pmd_populate() to use that page frame as a page table.
Function pmd_populate() uses macro set_pmd() that expands to function
xen_l2_entry_update() to update the (PGD) level2 page tables.

Thanks
Richard


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Re: Page table and memory management

[Keir Fraser]

On 3 Mar 2005, at 03:07, Richard wrote:

> How do I create a new page table ?
> How do I register a page frame that I own with XEN so that I can use
> it as a page table ?

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.
There are also mmu_op calls to pin a page to L1 or L2 type. This is 
particularly important for L2 pages to get good performance.

Before a page has pagetable type you can have a writable mapping of it 
and initiaqlise it directly. When Xen infers the pagetable type or you 
pin it to such a type, you must already have relinuqished and writable 
mappings you had of that page.

  -- Keir




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Re: Page table and memory management

[Richard]

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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Re: Page table and memory management

[Richard]

Hi,

I think the flow graph in the previous email was too big and might not
display properly:

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

Richard


On Thu, 3 Mar 2005 17:00:17 -0500, Richard <judicator3@gmail.com> wrote:
> 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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Xen hangs while booting Domain 0

[Dhawan, Puneet]

Hi

I am trying to boot Xen kernel 2.6.11rc5bk4 from SUSE on a Dell 2850 machine 
with SUSE 9.2 Professional as base OS. I have noticed that if I plug in an 
Ethernet or a fiber channel connection to the box, the boot process hangs 
after loading the kernel! 

Any suggestions..?

-Puneet.




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Re: Page table and memory management

[Christian Limpach]

On Thu, 3 Mar 2005 17:00:17 -0500, Richard <judicator3@gmail.com> wrote:
> > 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.

The L1 type gets set in get_page_type, when the page's type count is
zero and the page was of a different type until then (code path with
the "On type change" comment).  For this to work, the type count needs
to be zero which it isn't when there is already a writeable mapping to
the page.

You should use a debug=y build of xen, it will tell you why and when
your attempt to use a new L1 page fails.

> 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().

No, you have to make sure the page is not otherwise used, that there
are at most read-only mappings to it.

    christian


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Re: Page table and memory management

[Keir Fraser]

On 3 Mar 2005, at 22:00, Richard wrote:

> 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().
>
get_page_from_l2e() will do a get_page_type() on your new page, 
asserting type PGT_l1_pagetable. If the page was previously untyped or 
was already an L1 pagetable then the call will succeed, else it will 
fail.

  -- Keir
]



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