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[Xen-devel] Re: [PATCH 2/4] x86,xen: introduce x86_init.mapping.pagetabl

On Wed, 13 Apr 2011, H. Peter Anvin wrote:
> On 04/12/2011 04:19 AM, stefano.stabellini@xxxxxxxxxxxxx wrote:
> > From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > 
> > Introduce a new x86_init hook called pagetable_reserve that during the
> > initial memory mapping is used to reserve a range of memory addresses for
> > kernel pagetable usage.
> > 
> > On native it just calls memblock_x86_reserve_range while on xen it also
> > takes care of setting the spare memory previously allocated
> > for kernel pagetable pages from RO to RW, so that it can be used for
> > other purposes.
> > 
> What are the *semantics* of this hook?
> Hooks are insanely nasty if they are just defined by a particular code
> flow, as evidenced by the royal mess called paravirt_ops.

I hope that the other email I have just sent clarifies the purpose of
the hook.
I admit that as it is it wouldn't find much usage outside
Maybe adding the corresponding hook to allocate the initial kernel
pagetable pages would help generalizing it.  Or maybe we just need a
better comment in the code:

/* Reserve the kernel pagetable pages we used and free the other ones so
 * that they can be reused for other purposes.
 * On native it just means calling memblock_x86_reserve_range, on Xen it
 * also means marking RW the pagetable pages that we allocated before
 * but that haven't been used here.
if (!after_bootmem && pgt_buf_end > pgt_buf_start)

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