xen/mmu: Recycle the Xen provided L4, L3, and L2 pages

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]
[v4: Add more comments]
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

1 files changed, 35 insertions, 7 deletions

diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index a59070b09055..b44e6a88ea74 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 = pt_base + xen_start_info->nr_pt_frames;
+
         /* 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 +1804,26 @@ 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 .. So for dom0 we can only
+         * rip out the [L4] (pgd), but for guests we shave off three pages.
+         */
+        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);