Messages in this thread | | | Date | Mon, 16 May 2011 11:23:14 +0100 | From | Stefano Stabellini <> | Subject | Re: Xen MMU's requirement to pin pages RO and initial_memory_mapping. |
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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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