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xen-devel
[Xen-devel] Re: Xen MMU's requirement to pin pages RO and initial_memory
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To: |
Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> |
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Subject: |
[Xen-devel] Re: Xen MMU's requirement to pin pages RO and initial_memory_mapping. |
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From: |
Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx> |
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Date: |
Mon, 16 May 2011 11:23:14 +0100 |
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Cc: |
"jeremy@xxxxxxxx" <jeremy@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Stefano Stabellini <Stefano.Stabellini@xxxxxxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>, "hpa@xxxxxxxxx" <hpa@xxxxxxxxx>, "hpa@xxxxxxxxxxxxxxx" <hpa@xxxxxxxxxxxxxxx>, "yinghai@xxxxxxxxxx" <yinghai@xxxxxxxxxx> |
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Mon, 16 May 2011 03:22:16 -0700 |
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Alpine 2.00 (DEB 1167 2008-08-23) |
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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