Xen devel - May 2006 - Unable to start PAE Mini-OS from 0x0

I am trying to start PAE Mini-OS from 0x0 so that I can get access to
all 4GB. But it is failing in the builder (line 838 xc_linux_build.c)
when it tries to pin the L3 table with the following error below.
Further more, I am not able to bring up Mini-OS from any other address
other than 0xc0000000.

Is there a restriction on this in i386 PAE? Could someone please explain
why?

Thanks,
Aravindh Puthiyaparambil 
Xen Development Team 
Unisys, Tredyffrin PA

(XEN) DOM0: (file=mm.c, line=734) PAE L3 3rd slot is empty
(XEN) DOM0: (file=mm.c, line=918) Failure in alloc_l3_table: entry 4
(XEN) DOM0: (file=mm.c, line=1567) Error while validating mfn bd93b (pfn
e0c4) for type 60000000: caf=80000002 taf=60000001
(XEN) DOM0: (file=mm.c, line=1839) Error while pinning mfn bd93b

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On 19 May 2006, at 22:35, Puthiyaparambil, Aravindh wrote:
> I am trying to start PAE Mini-OS from 0x0 so that I can get access to
> all 4GB. But it is failing in the builder (line 838 xc_linux_build.c)
> when it tries to pin the L3 table with the following error below.
> Further more, I am not able to bring up Mini-OS from any other address
> other than 0xc0000000.
>
> Is there a restriction on this in i386 PAE? Could someone please 
> explain
> why?
Xen absolutely requires that the 4th PAE pgdir slot is filled as Xen 
hijacks entries in the pmd to map itself. In fact the builder should 
probably simply allocate 4 pmd pages always, and always fill all 4 
slots of the top-level pgdir when running on PAE.

  -- Keir


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This is fixed in a patch I submitted a week or so ago, which has not
yet been commented on or accepted.  Could someone please review
the patch and let me know what is deficient.

If it is acceptable I''d like to see it placed in both testing and
unstable.

-Bruce
>>> Keir Fraser <Keir.Fraser@cl.cam.ac.uk> 05/20/06 2:30 AM
>>>
On 19 May 2006, at 22:35, Puthiyaparambil, Aravindh wrote:
> I am trying to start PAE Mini-OS from 0x0 so that I can get access to
> all 4GB. But it is failing in the builder (line 838 xc_linux_build.c)
> when it tries to pin the L3 table with the following error below.
> Further more, I am not able to bring up Mini-OS from any other address
> other than 0xc0000000.
>
> Is there a restriction on this in i386 PAE? Could someone please 
> explain
> why?
Xen absolutely requires that the 4th PAE pgdir slot is filled as Xen 
hijacks entries in the pmd to map itself. In fact the builder should 
probably simply allocate 4 pmd pages always, and always fill all 4 
slots of the top-level pgdir when running on PAE.

  -- Keir


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http://lists.xensource.com/xen-devel


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> This is fixed in a patch I submitted a week or so ago, which has not
> yet been commented on or accepted.  Could someone please review
> the patch and let me know what is deficient.
> 
> If it is acceptable I''d like to see it placed in both testing and
> unstable.
With changeset 10053, I am able to get Mini-OS PAE to come up from 0x0.
I am noticing that number of pagetable frames created by the builder is
5 if virtual start is below 0xc0000000 and 4 if higher than 0xc0000000
for the same amount of memory (32MB). Why the difference in the number
of pagetable frames? Is it because of the extra mid-level page directory
for Xen?

Thanks,
Aravindh

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On 21 May 2006, at 23:53, Puthiyaparambil, Aravindh wrote:
> With changeset 10053, I am able to get Mini-OS PAE to come up from 0x0.
> I am noticing that number of pagetable frames created by the builder is
> 5 if virtual start is below 0xc0000000 and 4 if higher than 0xc0000000
> for the same amount of memory (32MB). Why the difference in the number
> of pagetable frames? Is it because of the extra mid-level page 
> directory
> for Xen?
Yes. We always create a final mid-level page directory even if not 
required for guest mappings.

  -- Keir


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> Yes. We always create a final mid-level page directory even if not
> required for guest mappings.
So in the Mini-OS pagetable builder, I should NOT create and pin a new
L2 frame for 0xc0000000 but only create the L1 frames and entries. Am I
right?

Thanks,
Aravindh

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