[Xen-devel] [PATCH v6] x86/setup: properly update PTEs if src/dst overlaps when relocating Xen image

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

Commit 0d31d16 (x86/setup: do not relocate Xen over current Xen image
placement) disallowed src/dst images overlaps when relocating Xen image.
Though it deliberately allowed destination region between __image_base__
and (__image_base__ + XEN_IMG_OFFSET) overlaps with the end of source
image. And here is the problem. If anything between __page_tables_start
and __page_tables_end in source image lands in the overlap then some or
even all page table entries may not be updated. This usually means boom
in early boot which will be difficult to the investigate. So, I think
that we have three choices to fix the issue:
  - drop XEN_IMG_OFFSET from
    if ( (end > s) && (end - reloc_size + XEN_IMG_OFFSET >= __pa(_end)) )
  - add XEN_IMG_OFFSET to xen_phys_start in PFN_DOWN(xen_phys_start)
    used in loops as one of conditions and replace ">" with ">=",
  - change PFN_DOWN(xen_phys_start) to PFN_DOWN(xen_remap_end_pfn)
    proposed in earlier version of this patch.

This patch implements the second option. This way we still allow source
and destination partial overlap as described above but PTEs are properly
updated now.

Signed-off-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
---
 xen/arch/x86/setup.c |   16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index b2baee3..f034a31 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -1012,6 +1012,7 @@ void __init noreturn __start_xen(unsigned long mbi_p)
             l3_pgentry_t *pl3e;
             l2_pgentry_t *pl2e;
             int i, j, k;
+            unsigned long pte_update_limit;
 
             /* Select relocation address. */
             e = end - reloc_size;
@@ -1019,6 +1020,15 @@ void __init noreturn __start_xen(unsigned long mbi_p)
             bootsym(trampoline_xen_phys_start) = e;
 
             /*
+             * All PTEs pointing above that address are not candidates for
+             * relocation. Due to possibility of partial overlap of the end
+             * of source image and the beginning of region for destination
+             * image some PTEs may point to addresses in
+             * range [e, e + XEN_IMG_OFFSET).
+             */
+            pte_update_limit = PFN_DOWN(e + XEN_IMG_OFFSET);
+
+            /*
              * Perform relocation to new physical address.
              * Before doing so we must sync static/global data with main memory
              * with a barrier(). After this we must *not* modify static/global
@@ -1041,7 +1051,7 @@ void __init noreturn __start_xen(unsigned long mbi_p)
                     /* Not present, 1GB mapping, or already relocated? */
                     if ( !(l3e_get_flags(*pl3e) & _PAGE_PRESENT) ||
                          (l3e_get_flags(*pl3e) & _PAGE_PSE) ||
-                         (l3e_get_pfn(*pl3e) > PFN_DOWN(xen_phys_start)) )
+                         (l3e_get_pfn(*pl3e) >= pte_update_limit) )
                         continue;
                     *pl3e = l3e_from_intpte(l3e_get_intpte(*pl3e) +
                                             xen_phys_start);
@@ -1051,7 +1061,7 @@ void __init noreturn __start_xen(unsigned long mbi_p)
                         /* Not present, PSE, or already relocated? */
                         if ( !(l2e_get_flags(*pl2e) & _PAGE_PRESENT) ||
                              (l2e_get_flags(*pl2e) & _PAGE_PSE) ||
-                             (l2e_get_pfn(*pl2e) > PFN_DOWN(xen_phys_start)) )
+                             (l2e_get_pfn(*pl2e) >= pte_update_limit) )
                             continue;
                         *pl2e = l2e_from_intpte(l2e_get_intpte(*pl2e) +
                                                 xen_phys_start);
@@ -1075,7 +1085,7 @@ void __init noreturn __start_xen(unsigned long mbi_p)
                 unsigned int flags;
 
                 if ( !(l2e_get_flags(*pl2e) & _PAGE_PRESENT) ||
-                     (l2e_get_pfn(*pl2e) > PFN_DOWN(xen_phys_start)) )
+                     (l2e_get_pfn(*pl2e) >= pte_update_limit) )
                     continue;
 
                 if ( !using_2M_mapping() )
-- 
1.7.10.4


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