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[Xen-devel] [PATCH 10/16] xen: clear reserved bits in l3 entries given in the initial pagetables

From: Ian Campbell <ian.campbell@xxxxxxxxxx>

In native PAE, the only flag that may be legitimately set in an L3
entry is Present.  When Xen grafts the top-level PAE L3 pagetable
entries into the L4 pagetable, it must also set the other permissions
flags so that the mapped pages are actually accessible.

However, due to a bug in the hypervisor, it validates update to the L3
entries as formal PAE entries, so it will refuse to validate these
entries with the extra bits requires for 4-level pagetables.

This patch simply masks the entries back to the bare PAE level,
leaving Xen to add whatever bits it feels are necessary.

[ Impact: workaround Xen bug in 32-on-64 dom0 ]

Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@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 05cfd30..85d9d18 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -1835,6 +1835,7 @@ __init pgd_t *xen_setup_kernel_pagetable(pgd_t *pgd,
                                         unsigned long max_pfn)
        pmd_t *kernel_pmd;
+       int i;
        max_pfn_mapped = PFN_DOWN(__pa(xen_start_info->pt_base) +
                                  xen_start_info->nr_pt_frames * PAGE_SIZE +
@@ -1846,6 +1847,20 @@ __init pgd_t *xen_setup_kernel_pagetable(pgd_t *pgd,
        xen_map_identity_early(level2_kernel_pgt, max_pfn);
        memcpy(swapper_pg_dir, pgd, sizeof(pgd_t) * PTRS_PER_PGD);
+       /*
+        * When running a 32 bit domain 0 on a 64 bit hypervisor a
+        * pinned L3 (such as the initial pgd here) contains bits
+        * which are reserved in the PAE layout but not in the 64 bit
+        * layout. Unfortunately some versions of the hypervisor
+        * (incorrectly) validate compat mode guests against the PAE
+        * layout and hence will not allow such a pagetable to be
+        * pinned by the guest. Therefore we mask off only the PFN and
+        * Present bits of the supplied L3.
+        */
+       for (i = 0; i < PTRS_PER_PGD; i++)
+               swapper_pg_dir[i].pgd &= (PTE_PFN_MASK | _PAGE_PRESENT);
                        __pgd(__pa(level2_kernel_pgt) | _PAGE_PRESENT));

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