Re: Xen MMU's requirement to pin pages RO andinitial_memory_mapping.

[Stefano Stabellini]

On Fri, 13 May 2011, Konrad Rzeszutek Wilk wrote:
> aka, remove the hack added by git commit 609cfda586c7fe3e5d1a02c51edb587506294167
> (Merge branch 'stable/bug-fixes-for-rc5' of git://git.kernel.org/../xen)
> 
> One idea that is on the table was proposed by Yinghai:
> "Xen should set RAM for page-table to RO after init_memory mapping."
> 
> In other words, don't do the magic 'mask_rw_pte' when set_pte is called.
> (and don't do the calls to make_lowmem_page_readonly when allocating
> the PTE table, nor PMD, nor PUD) - I think?
> 
> But instead do:
>  a). when we load the cr3? We could go through the whole pagetable and
>      set the RO as we need?
>  b). when we are finished with the creation of a page table? So similary 
>      to the point above - don't set the RO on the pages until we have completed
>      the full creation of the page.
>  c). when post_allocator_start is called?

I think c) is too late because there might be allocations or at least
memblock allocations after init_memory_mapping and before
post_allocator_start.


>  d). other ideas?
> 
> But I vaguelly recall that we are using the page table as we are adding in the
> entries. And that we are pinning them as well. Perhaps the trigger to scan the
> pagetable and set them to RO should be done .. at what point? When the PUD/PMD
> allocations are done? And when PUD/PMD are set?
> 

Setting the pagetable pages RO after init_memory_mapping is difficult
because pagetable pages have to be set RO before becoming pagetable
pages.
They become pagetable pages when:

- they are explicitly pinned by pin_pagetable_pfn

- they are hooked into the current pagetable

Like you wrote, considering that the x86_64 version of
kernel_physical_mapping_init hooks the pagetable pages into the
currently used pagetable, it wouldn't be possible to mark the pagetable
pages RO after init_memory_mapping.
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