1 files changed, 18 insertions, 0 deletions

diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
index e2d62d697b5d..8971a26d21ab 100644
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -432,6 +432,24 @@ char * __init xen_memory_setup(void)
          *  - mfn_list
          *  - xen_start_info
          * See comment above "struct start_info" in <xen/interface/xen.h>
+         * We tried to make the the memblock_reserve more selective so
+         * that it would be clear what region is reserved. Sadly we ran
+         * in the problem wherein on a 64-bit hypervisor with a 32-bit
+         * initial domain, the pt_base has the cr3 value which is not
+         * neccessarily where the pagetable starts! As Jan put it: "
+         * Actually, the adjustment turns out to be correct: The page
+         * tables for a 32-on-64 dom0 get allocated in the order "first L1",
+         * "first L2", "first L3", so the offset to the page table base is
+         * indeed 2. When reading xen/include/public/xen.h's comment
+         * very strictly, this is not a violation (since there nothing is said
+         * that the first thing in the page table space is pointed to by
+         * pt_base; I admit that this seems to be implied though, namely
+         * do I think that it is implied that the page table space is the
+         * range [pt_base, pt_base + nt_pt_frames), whereas that
+         * range here indeed is [pt_base - 2, pt_base - 2 + nt_pt_frames),
+         * which - without a priori knowledge - the kernel would have
+         * difficulty to figure out)." - so lets just fall back to the
+         * easy way and reserve the whole region.
          */
         memblock_reserve(__pa(xen_start_info->mfn_list),
                          xen_start_info->pt_base - xen_start_info->mfn_list);