Re: [Xen-devel] pagetable maps itself

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 13 Mar 2005, at 15:21, David Becker wrote:

> I'm trying to put the shared_info page entry(0x001b6007) in
> a pte at 0xc0116404 and get
> (XEN) DOM77: (file=memory.c, line=1821) PTWR failure! Pagetable maps 
> itself at c0116404
> Doubtless this is operator error, but I'm at a loss as to what my 
> error is.
> I am confident the pte address is correct and that the domain build 
> says the
> share_info mfn is 0x1b6.
>
> The pgd page entry for 0xc0116404 is 0x01c28067, which also appears OK.
>
> I don't understand how my table is mapping itself when I write that 
> pte.
It's a current weakness of the writable pagetable implementation -- if 
you try to directly write to a pagetable that maps itself then the 
usual wrpt implementation cannot work. It wants to temporarily make the 
page a non-pt page and make it writable: but it can't do both (think 
about it).
I will be fixing this in the unstable tree in the next few days, using 
my new instruction emulator to 'fill in' this unfortunate corner case 
and a few others. Shorter term, please use the batched interface 
(do_mmu_update) to add the mapping. I use that in preference to 
writable pagetables to update most kernel mappings in Linux right now, 
to avoid just the problem that you have just hit.
 -- Keir



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