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

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

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@oracle.com>
---
 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