[PATCH v2] xen/pdx: fix offset-compression merge of a contained range

[Weiqi Wang <coolhaoyt@xxxxxxxxx>]

From: Weiqi Wang <weiqi.wang-2@xxxxxxxxxxxxxxxxxxxxxxxxx>

When sorting and merging overlapping ranges in
pfn_pdx_compression_setup(), the merged range is set to end where the
second range ends.  If the second range is fully contained in the first,
this truncates the first range, and the tail of it is then neither
compressible nor translated correctly.

Keep the end of the merged range as the maximum of both ends.

On x86 the ranges come from the SRAT memory affinity entries, and
overlapping entries for the same node are tolerated with a warning by the
NUMA code.  The caller's subsequent coverage check catches the truncated
range, so the effect is that PDX compression is disabled with a "RAM
region ... not covered" message rather than memory being mistranslated.

Add a test case that fails without this change.

Found with the ESBMC bounded model checker.  The counterexample was
confirmed by running it natively against the unmodified code.

Fixes: c5c45bcbd6a1 ("pdx: introduce a new compression algorithm based on 
region offsets")
Assisted-by: Claude Code:claude-opus-5-5 # finding the issue with ESBMC, patch 
creation
Signed-off-by: Weiqi Wang <weiqi.wang-2@xxxxxxxxxxxxxxxxxxxxxxxxx>
---

Notes:
    Changes in v2:
    - clarify in the title that this is the offset-compression instance (Jan)
    - parenthesise the test multiplications against the binary ORs (Jan)
    
    Also seen in a real boot: the hypervisor alone under QEMU (pc, 8 GiB),
    built from defconfig, with no -numa and a hand-built SRAT passed with
    -acpitable.  Memory affinity entries, all PXM 0:
    
      [0, 3G)  [4G, 9G)  [5G, 6G)  [1T, 1T+1G)
    
    The third entry lies inside the second.  The NUMA code prints "overlaps
    with itself" for it and accepts it.  Without this patch:
    
      (XEN) PFN compression using lookup table shift 23 and region size 0x200000
      (XEN)  range   0 [0000000000000, 000000017ffff] PFN IDX   0 : 
0000000000000
      (XEN)  range   1 [0000010000000, 000001003ffff] PFN IDX  32 : 
000000fe00000
      (XEN) PFN compression disabled, RAM region [0x100000000, 0x23fffffff] not 
covered
    
    With it, the same output as without the third entry:
    
      (XEN) PFN compression using lookup table shift 28 and region size 0x400000
      (XEN)  range   0 [0000000000000, 000000023ffff] PFN IDX   0 : 
0000000000000
      (XEN)  range   1 [0000010000000, 000001003ffff] PFN IDX   1 : 
000000fc00000

 tools/tests/pdx/test-pdx.c | 12 ++++++++++++
 xen/common/pdx.c           |  5 +++--
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/tools/tests/pdx/test-pdx.c b/tools/tests/pdx/test-pdx.c
index 4de8d43d86..8d271a488f 100644
--- a/tools/tests/pdx/test-pdx.c
+++ b/tools/tests/pdx/test-pdx.c
@@ -87,6 +87,18 @@ int main(int argc, char **argv)
             },
             .compress = true,
         },
+        /* Range contained in a previous one. */
+        {
+            .ranges = {
+                { .start =                                                 0,
+                  .end   =                          ((1UL << MAX_ORDER) * 1) },
+                { .start = (1UL << (MAX_ORDER * 2)) |                      0,
+                  .end   = (1UL << (MAX_ORDER * 2)) | ((1UL << MAX_ORDER) * 4) 
},
+                { .start = (1UL << (MAX_ORDER * 2)) | ((1UL << MAX_ORDER) * 1),
+                  .end   = (1UL << (MAX_ORDER * 2)) | ((1UL << MAX_ORDER) * 2) 
},
+            },
+            .compress = true,
+        },
 #endif
         /* PDX compression, 2 ranges covered by the lower mask. */
         {
diff --git a/xen/common/pdx.c b/xen/common/pdx.c
index e7e16e193e..23655ef3bd 100644
--- a/xen/common/pdx.c
+++ b/xen/common/pdx.c
@@ -393,8 +393,9 @@ bool __init pfn_pdx_compression_setup(paddr_t base)
              (ranges[i - 1].base_pfn + ranges[i - 1].pages) )
             continue;
 
-        ranges[i - 1].pages = ranges[i].base_pfn + ranges[i].pages -
-                              ranges[i - 1].base_pfn;
+        ranges[i - 1].pages = max(ranges[i - 1].pages,
+                                  ranges[i].base_pfn + ranges[i].pages -
+                                  ranges[i - 1].base_pfn);
 
         if ( i + 1 < nr_ranges )
             memmove(&ranges[i], &ranges[i + 1],
-- 
2.34.1