[xen stable-4.21] xen/pdx: fix off-by-one index in offset mask calculation

[patchbot@xxxxxxx]

commit 98b54de20a043c8ee1b49d34d4b0ebf7d3eff478
Author:     Roger Pau Monné <roger.pau@xxxxxxxxxx>
AuthorDate: Tue Jun 30 14:52:42 2026 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Jun 30 14:52:42 2026 +0200

    xen/pdx: fix off-by-one index in offset mask calculation
    
    Adjust the mask calculation in case the last range is merged with the
    previous one, as then the mask must be calculated from the previous range,
    which the current one has been merged into.
    
    Instead of fixing the off-by-one in place, move the calculation of the bit
    change mask to the next loop, after the ranges have been merged.  This
    simplifies the logic by consolidating mask calculation in a single place,
    possibly making it less error prone in the future.
    
    Also add a test case that triggers the bug being fixed by this commit.
    
    Fixes: c5c45bcbd6a1 ("pdx: introduce a new compression algorithm based on 
region offsets")
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    master commit: edf942201dd112dee6b040f7eb0bf2b8a7608047
    master date: 2026-06-09 11:43:24 +0200
---
 tools/tests/pdx/test-pdx.c | 14 ++++++++++++++
 xen/common/pdx.c           | 13 ++++++-------
 2 files changed, 20 insertions(+), 7 deletions(-)

diff --git a/tools/tests/pdx/test-pdx.c b/tools/tests/pdx/test-pdx.c
index eefd54c768..8dd4ee9c0a 100644
--- a/tools/tests/pdx/test-pdx.c
+++ b/tools/tests/pdx/test-pdx.c
@@ -191,6 +191,20 @@ int main(int argc, char **argv)
             },
             .compress = false,
         },
+        /*
+         * Dell R740, dual socket.  Merging of ranges causes mask differences
+         * in PDX offset mode.  Useful for checking mask calculations.
+         */
+        {
+            .ranges = {
+                { .start = 0x0000000UL, .end = 0x0080000UL },
+                { .start = 0x0100000UL, .end = 0x3070000UL },
+                { .start = 0x3070000UL, .end = 0x3870000UL },
+                { .start = 0x3870000UL, .end = 0x6870000UL },
+                { .start = 0x6870000UL, .end = 0x7070000UL },
+            },
+            .compress = false,
+        },
     };
     int ret_code = EXIT_SUCCESS;
 
diff --git a/xen/common/pdx.c b/xen/common/pdx.c
index 7e070ff962..e7e16e193e 100644
--- a/xen/common/pdx.c
+++ b/xen/common/pdx.c
@@ -391,10 +391,7 @@ bool __init pfn_pdx_compression_setup(paddr_t base)
         if ( !i ||
              ranges[i].base_pfn >=
              (ranges[i - 1].base_pfn + ranges[i - 1].pages) )
-        {
-            mask |= pdx_region_mask(ranges[i].base_pfn, ranges[i].pages);
             continue;
-        }
 
         ranges[i - 1].pages = ranges[i].base_pfn + ranges[i].pages -
                               ranges[i - 1].base_pfn;
@@ -402,19 +399,21 @@ bool __init pfn_pdx_compression_setup(paddr_t base)
         if ( i + 1 < nr_ranges )
             memmove(&ranges[i], &ranges[i + 1],
                     (nr_ranges - (i + 1)) * sizeof(ranges[0]));
-        else /* last range */
-            mask |= pdx_region_mask(ranges[i].base_pfn, ranges[i].pages);
         nr_ranges--;
         i--;
     }
 
     /*
-     * Populate a mask with the non-equal bits of the different ranges, do this
-     * to calculate the maximum PFN shift to use as the lookup table index.
+     * Populate two masks: one with the non-equal bits of the different ranges,
+     * another with the bits that change inside ranges.  Do this to calculate
+     * the maximum PFN shift to use as the lookup table index.
      */
     for ( i = 0; i < nr_ranges; i++ )
+    {
+        mask |= pdx_region_mask(ranges[i].base_pfn, ranges[i].pages);
         for ( unsigned int j = i + 1; j < nr_ranges; j++ )
             idx_mask |= ranges[i].base_pfn ^ ranges[j].base_pfn;
+    }
     /* Mask out the bits that change inside of ranges. */
     idx_mask &= ~mask;
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.21