[Xen-devel] [PATCH 3/5] xen/mmu: Provide comments describing the _kva and _va aliasing issue

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

And also explain how the pagetables are being grafted on.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
 arch/x86/xen/mmu.c |   15 +++++++++++++++
 1 files changed, 15 insertions(+), 0 deletions(-)

diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 4ac21a4..b0b72e0 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -1734,19 +1734,34 @@ 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 __kva space! */
        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);
-- 
1.7.7.6


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