Re: Q:pt_base in COMPAT mode offset by two pages. Was:Re: [Xen-devel] [PATCH 02/11] xen/x86: Use memblock_reserve for sensitive areas.

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

> > +	/* Under 64-bit hypervisor with a 32-bit domain, the hypervisor
> > +	 * offsets the pt_base by two pages. Hence the reservation that is done
> > +	 * in mmu.c misses two pages. We correct it here if we detect this. */
> > +	if (last_phys < __pa(xen_start_info->pt_base))
> > +		memblock_reserve(last_phys, __pa(xen_start_info->pt_base) - last_phys);
> >  }
> 
> What are these two pages used for? They are not documented in xen.h, why
> should we reserve them?
> 
> After all we still have:
> 
> memblock_reserve(PFN_PHYS(pt_base), (pt_end - pt_base) * PAGE_SIZE);
> 
> that should protect what we are interested in anyway...

You are looking at the x86_64 piece of code. This issue only appears
on 32-bit which was not modified by my patches and has:

2003         memblock_reserve(__pa(xen_start_info->pt_base),
2004                          xen_start_info->nr_pt_frames * PAGE_SIZE);

and as I found out, the pt_base is wrong. The cr3 we load and use is actually
two pages back!