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    Subject[PATCH 3/6] xen/mmu: Recycle the Xen provided L4, L3, and L2 pages
    Date
    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]
    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..de4b8fd 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 __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)));
    +
    /* 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]);
    +
    + /* 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


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