Re: [PATCH v4 1/2] XEN/X86: Improve semantic support forx86_init.mapping.pagetable_reserve

[Thomas Gleixner]

On Fri, 24 Aug 2012, Stefano Stabellini wrote:
> On Fri, 24 Aug 2012, Thomas Gleixner wrote:
> > I looked at the whole pgt_buf_* mess and it's amazingly stupid. We
> > could avoid all that dance and make all of that pgt_buf_* stuff static
> > and provide proper accessor functions and hand start, end, top to the
> > reserve function instead of fiddling with global variables all over
> > the place. That'd be a real cleanup and progress.
> > 
> > But we can't do that easily. And why? Because XEN is making magic
> > decisions based on those globals in mask_rw_pte().
> > 
> > 	/*
> > 	 * If the new pfn is within the range of the newly allocated
> > 	 * kernel pagetable, and it isn't being mapped into an
> > 	 * early_ioremap fixmap slot as a freshly allocated page, make sure
> > 	 * it is RO.
> > 	 */
> > 	if (((!is_early_ioremap_ptep(ptep) &&
> > 			pfn >= pgt_buf_start && pfn < pgt_buf_top)) ||
> > 			(is_early_ioremap_ptep(ptep) && pfn != (pgt_buf_end - 1)))
> > 
> > This comment along with the implementation is really a master piece of
> > obfuscation. Let's see what this is doing. RO is enforced when:
> > 
> > This is not an early ioreamp AND
> > 
> >       pfn >= pgt_buf_start && pfn < pgt_buf_top
> > 
> > So why is this checking pgt_buf_top? The early stuff is installed
> > within pgt_buf_start and pgt_buf_end. Anything which is >=
> > pgt_buf_end at this point is completely wrong.
> 
> Unfortunately pgt_buf_end only marks the current end of the pagetable
> pages (pgt_buf_end keeps increasing during
> kernel_physical_mapping_init).  However at some point
> kernel_physical_mapping_init is going to start mapping the pagetable
> pages themselves, when that happens some of them are not pagetable pages
> yet (pgt_buf_end <= page < pgt_buf_top) but they are going to be in the
> near future.

And how exactly are they allocated between from pgt_buf w/o increasing
pgt_buf_end ?

Thanks,

	tglx
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