xen/mmu: Provide comments describing the _ka and _va aliasing issue

Which is that the level2_kernel_pgt (__ka virtual addresses)
and level2_ident_pgt (__va virtual address) contain the same
PMD entries. So if you modify a PTE in __ka, it will be reflected
in __va (and vice-versa).

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 4ac21a4c6da4..6ba610098dd9 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -1734,19 +1734,36 @@ void __init xen_setup_kernel_pagetable(pgd_t *pgd, unsigned long max_pfn)
         init_level4_pgt[0] = __pgd(0);
 
         /* Pre-constructed entries are in pfn, so convert to mfn */
+        /* L4[272] -> level3_ident_pgt
+         * L4[511] -> level3_kernel_pgt */
         convert_pfn_mfn(init_level4_pgt);
+
+        /* L3_i[0] -> level2_ident_pgt */
         convert_pfn_mfn(level3_ident_pgt);
+        /* L3_k[510] -> level2_kernel_pgt
+         * L3_i[511] -> level2_fixmap_pgt */
         convert_pfn_mfn(level3_kernel_pgt);
 
+        /* We get [511][511] and have Xen's version of level2_kernel_pgt */
         l3 = m2v(pgd[pgd_index(__START_KERNEL_map)].pgd);
         l2 = m2v(l3[pud_index(__START_KERNEL_map)].pud);
 
+        /* 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
+         * it will be also modified in the __ka space! (But if you just
+         * modify the PMD table to point to other PTE's or none, then you
+         * are OK - which is what cleanup_highmap does) */
         memcpy(level2_ident_pgt, l2, sizeof(pmd_t) * PTRS_PER_PMD);
+        /* Graft it onto L4[511][511] */
         memcpy(level2_kernel_pgt, l2, sizeof(pmd_t) * PTRS_PER_PMD);
 
+        /* Get [511][510] and graft that in level2_fixmap_pgt */
         l3 = m2v(pgd[pgd_index(__START_KERNEL_map + PMD_SIZE)].pgd);
         l2 = m2v(l3[pud_index(__START_KERNEL_map + PMD_SIZE)].pud);
         memcpy(level2_fixmap_pgt, l2, sizeof(pmd_t) * PTRS_PER_PMD);
+        /* Note that we don't do anything with level1_fixmap_pgt which
+         * we don't need. */
 
         /* Set up identity map */
         xen_map_identity_early(level2_ident_pgt, max_pfn);