Re: [Xen-devel] [PATCH 07/11] xen/mmu: Recycle the Xen provided L4, L3, and L2 pages

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

On Fri, Aug 17, 2012 at 07:07:28PM +0100, Stefano Stabellini wrote:
> On Thu, 16 Aug 2012, Konrad Rzeszutek Wilk wrote:
> > As we are not using them. We end up only using the L1 pagetables
> > and grafting those to our page-tables.
> > 
> > [v1: Per Stefano's suggestion squashed two commits]
> > [v2: Per Stefano's suggestion simplified loop]
> > [v3: Fix smatch warnings]
> > Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> > ---
> >  arch/x86/xen/mmu.c |   40 +++++++++++++++++++++++++++++++++-------
> >  1 files changed, 33 insertions(+), 7 deletions(-)
> > 
> > diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
> > index a59070b..bd92c82 100644
> > --- a/arch/x86/xen/mmu.c
> > +++ b/arch/x86/xen/mmu.c
> > @@ -1708,7 +1708,20 @@ static void convert_pfn_mfn(void *v)
> >  	for (i = 0; i < PTRS_PER_PTE; i++)
> >  		pte[i] = xen_make_pte(pte[i].pte);
> >  }
> > -
> > +static void __init check_pt_base(unsigned long *pt_base, unsigned long *pt_end,
> > +				 unsigned long addr)
> > +{
> > +	if (*pt_base == PFN_DOWN(__pa(addr))) {
> > +		set_page_prot((void *)addr, PAGE_KERNEL);
> > +		clear_page((void *)addr);
> > +		(*pt_base)++;
> > +	}
> > +	if (*pt_end == PFN_DOWN(__pa(addr))) {
> > +		set_page_prot((void *)addr, PAGE_KERNEL);
> > +		clear_page((void *)addr);
> > +		(*pt_end)--;
> > +	}
> > +}
> >  /*
> >   * Set up the initial kernel pagetable.
> >   *
> > @@ -1724,6 +1737,9 @@ void __init xen_setup_kernel_pagetable(pgd_t *pgd, unsigned long max_pfn)
> >  {
> >  	pud_t *l3;
> >  	pmd_t *l2;
> > +	unsigned long addr[3];
> > +	unsigned long pt_base, pt_end;
> > +	unsigned i;
> >  
> >  	/* max_pfn_mapped is the last pfn mapped in the initial memory
> >  	 * mappings. Considering that on Xen after the kernel mappings we
> > @@ -1731,6 +1747,9 @@ void __init xen_setup_kernel_pagetable(pgd_t *pgd, unsigned long max_pfn)
> >  	 * set max_pfn_mapped to the last real pfn mapped. */
> >  	max_pfn_mapped = PFN_DOWN(__pa(xen_start_info->mfn_list));
> >  
> > +	pt_base = PFN_DOWN(__pa(xen_start_info->pt_base));
> > +	pt_end = PFN_DOWN(__pa(xen_start_info->pt_base + (xen_start_info->nr_pt_frames * PAGE_SIZE)));
> 

or just do:

	pt_end = pt_base + xen_start_info->nr_pt_frames;

> code style
> 
> >  	/* Zap identity mapping */
> >  	init_level4_pgt[0] = __pgd(0);
> >  
> > @@ -1749,6 +1768,9 @@ void __init xen_setup_kernel_pagetable(pgd_t *pgd, unsigned long max_pfn)
> >  	l3 = m2v(pgd[pgd_index(__START_KERNEL_map)].pgd);
> >  	l2 = m2v(l3[pud_index(__START_KERNEL_map)].pud);
> >  
> > +	addr[0] = (unsigned long)pgd;
> > +	addr[1] = (unsigned long)l3;
> > +	addr[2] = (unsigned long)l2;
> >  	/* Graft it onto L4[272][0]. Note that we creating an aliasing problem:
> >  	 * Both L4[272][0] and L4[511][511] have entries that point to the same
> >  	 * L2 (PMD) tables. Meaning that if you modify it in __va space
> > @@ -1782,20 +1804,24 @@ void __init xen_setup_kernel_pagetable(pgd_t *pgd, unsigned long max_pfn)
> >  	/* Unpin Xen-provided one */
> >  	pin_pagetable_pfn(MMUEXT_UNPIN_TABLE, PFN_DOWN(__pa(pgd)));
> >  
> > -	/* Switch over */
> > -	pgd = init_level4_pgt;
> > -
> >  	/*
> >  	 * At this stage there can be no user pgd, and no page
> >  	 * structure to attach it to, so make sure we just set kernel
> >  	 * pgd.
> >  	 */
> >  	xen_mc_batch();
> > -	__xen_write_cr3(true, __pa(pgd));
> > +	__xen_write_cr3(true, __pa(init_level4_pgt));
> >  	xen_mc_issue(PARAVIRT_LAZY_CPU);
> >  
> > -	memblock_reserve(__pa(xen_start_info->pt_base),
> > -			 xen_start_info->nr_pt_frames * PAGE_SIZE);
> > +	/* We can't that easily rip out L3 and L2, as the Xen pagetables are
> > +	 * set out this way: [L4], [L1], [L2], [L3], [L1], [L1] ...  for
> > +	 * the initial domain. For guests using the toolstack, they are in:
> > +	 * [L4], [L3], [L2], [L1], [L1], order .. */
> > +	for (i = 0; i < ARRAY_SIZE(addr); i++)
> > +		check_pt_base(&pt_base, &pt_end, addr[i]);
> 
> It is much clearer now, but if the comment is correct, doesn't it mean
> that we are going to be able to free pgd, l3 and l2 only in the non-dom0
> case?

And in dom0 case only PGD.

> If so it might be worth saying it explicitly.

OK.
> 
> Other than that, it is fine by me.
> 
> 
> > +	/* Our (by three pages) smaller Xen pagetable that we are using */
> > +	memblock_reserve(PFN_PHYS(pt_base), (pt_end - pt_base) * PAGE_SIZE);
> >  }
> >  #else	/* !CONFIG_X86_64 */
> >  static RESERVE_BRK_ARRAY(pmd_t, initial_kernel_pmd, PTRS_PER_PMD);
> > -- 
> > 1.7.7.6
> > 
> > 
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@lists.xen.org
> > http://lists.xen.org/xen-devel
> >