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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>
This commit is contained in:
Konrad Rzeszutek Wilk 2012-07-12 13:55:25 -04:00
parent 3699aad047
commit 4fac153a7a
1 changed files with 17 additions and 0 deletions
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17
arch/x86/xen/mmu.c
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@ -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);