Re: [PATCH 2/4] x86,xen: introduce x86_init.mapping.pagetable_reserve

[Stefano Stabellini <stefano.stabellini@eu.citrix.com>]

On Wed, 13 Apr 2011, H. Peter Anvin wrote:
> On 04/12/2011 04:19 AM, stefano.stabellini@eu.citrix.com wrote:
> > From: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
> > 
> > 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
init_memory_mapping.
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)
		x86_init.mapping.pagetable_reserve(PFN_PHYS(pgt_buf_start),
				PFN_PHYS(pgt_buf_end));