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[Xen-devel] Re: Xen MMU's requirement to pin pages RO and initial_memory_mapping.

On Mon, May 23, 2011 at 04:20:15PM +0100, Stefano Stabellini wrote:
> On Tue, 17 May 2011, Konrad Rzeszutek Wilk wrote:
> > On Tue, May 17, 2011 at 06:50:55PM +0100, Stefano Stabellini wrote:
> > > On Mon, 16 May 2011, Konrad Rzeszutek Wilk wrote:
> > > > > They become pagetable pages when:
> > > > > 
> > > > > - they are explicitly pinned by pin_pagetable_pfn
> > > > > 
> > > > > - they are hooked into the current pagetable
> > > > 
> > > > Ok, so could we use those two calls to trigger the pagetable walk
> > > > and mark them RO as appropiate? Which call sites are those? The
> > > > xen_set_pgd/xen_set_pud/xen_set_pmd ?
> > > 
> > > xen_alloc_pte_init and xen_alloc_pmd_init are the ones that mark the
> > > pagetable pages RO and pin them, calling make_lowmem_page_readonly and
> > > pin_pagetable_pfn.
> > > 
> > > alloc_pte/pmd are called right before hooking them into the pagetable;
> > > unfortunately that means that they fail at marking the pagetable pages
> > > RO: make_lowmem_page_readonly uses lookup_address to find the pte
> > > corresponding to a page, however at this point the pagetable pages are
> > > not mapped yet (usually they are not hooked but when they are hooked, the
> > > upper level pagetable page is not hooked), so lookup_address fails.
> > 
> > Right. We don't have to walk the hooked pagetable, I think. We are passed
> > in the PMD/PGD of the PFN and we could look at the content of that PFN.
> > Walk each entry in there and for those that are present, determine
> > if the page table it points to (whatever level it is) is RO. If not, mark
> > it RO. And naturally do it recursively to cover all levels.
> I am not sure what you mean.
> The interface is the following:
> void alloc_pte(struct mm_struct *mm, unsigned long pfn);
> pfn is the pagetable page's pfn, that has to be marked RO in all his mappings;
> mm is the mm_struct where this pagetable page is mapped.
> Except it is not true anymore because the pagetable page is not mapped
> yet in mm; so we cannot walk anything here, unfortunately.

I was thinking to "resolve" the pfn, and directly read from the pfn's the
entries. So not walking the mm_struct, but reading the raw data from the 
PFN page... but I that would not do much as alloc_pte is done _before_ that
pagetable is actually populated - so it has nothing in it.

> We could remember that we failed to mark this page RO so that the next
> time we try to write a pte that contains that address we know we have to
> mark it RO.
> But this approach is basically equivalent to the one we had before
> 2.6.39: we consider the range pgt_buf_start-pgt_buf_end a "published"
> range of pagetable pages that we have to mark RO.
> In fact it is affected by the same problem: after writing the ptes that
> map the range pgt_buf_start-pgt_buf_end, if we try to allocate a new
> pagetable page incrementing pgt_buf_end we fail because the new page is
> already marked RW in a pinned page.
> At the same time we cannot modify the pte to change the mapping to RO
> because lookup_address doesn't find it (the pagetable page containing
> the pte in question is not reachable from init_mm yet).

So.. why not do the raw walk of the PFN (and within this
"raw walk" ioremap the PFNs, and do a depth-first walk on the page-tables
do set them to RO) when it is being hooked up to the page-table?
Meaning - whatever trigger point is when we try to set a PUD in a PGD,
or PTE into a PMD. And naturally we can't walk the 'init_mm' as it
has not been hooked up yet (and it cannot as the page-tables have not
been set to RO).

> Unfortunately I cannot see an easy way to fix alloc_pte without making
> sure that the pfn passed as an argument is already mapped and the pte is
> reachable using lookup_address.

Lets ignore that for now.
> Alternatively we could come up with a new interface that properly
> publishes the pgt_buf_start-pgt_buf_top range, but it would still need a
> "free" function for the pgt_buf_end-pgt_buf_top range to be called after
> the initial mapping is complete.

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