[Xen-devel] [PATCH 4/7] xen/mmu: Recycle the Xen provided L4, L3, and L2 pages

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

As we are not using them. We end up only using the L1 pagetables
and grafting those to our page-tables.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
 arch/x86/xen/mmu.c |   38 ++++++++++++++++++++++++++++++++------
 1 files changed, 32 insertions(+), 6 deletions(-)

diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 48bdc9f..7f54b75 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -1724,6 +1724,9 @@ 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 +1734,9 @@ 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 +1755,9 @@ 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)l2;
+       addr[2] = (unsigned long)l3;
        /* 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
@@ -1791,12 +1800,29 @@ void __init xen_setup_kernel_pagetable(pgd_t *pgd, 
unsigned long max_pfn)
        __xen_write_cr3(true, __pa(init_level4_pgt));
        xen_mc_issue(PARAVIRT_LAZY_CPU);
 
-       /* Offset by one page since the original pgd is going bye bye */
-       memblock_reserve(__pa(xen_start_info->pt_base + PAGE_SIZE),
-                        (xen_start_info->nr_pt_frames * PAGE_SIZE) - 
PAGE_SIZE);
-       /* and also RW it so it can actually be used. */
-       set_page_prot(pgd, PAGE_KERNEL);
-       clear_page(pgd);
+       /* 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++) {
+               unsigned j;
+               /* No idea about the order the addr are in, so just do them 
twice. */
+               for (j = 0; j < ARRAY_SIZE(addr); j++) {
+                       if (pt_base == PFN_DOWN(__pa(addr[j]))) {
+                               set_page_prot((void *)addr[j], PAGE_KERNEL);
+                               clear_page((void *)addr[j]);
+                               pt_base++;
+
+                       }
+                       if (pt_end == PFN_DOWN(__pa(addr[j]))) {
+                               set_page_prot((void *)addr[j], PAGE_KERNEL);
+                               clear_page((void *)addr[j]);
+                               pt_end--;
+                       }
+               }
+       }
+       /* 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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