x86/mcheck: fix mca_allbanks updating

In mca_cap_init function,the mcabanks_alloc allocates and
initializes an mca_banks structure for managing MCA banks,
setting up a bank map and storing the specified or default number
of banks.

After this we will call mcabanks_set(i, mca_allbanks);
The mcabanks_set function sets a specific bit in the bank_map of
an mca_banks structure, provided the structure, its bank_map, and
the bit index are valid.

At the end, we will call
mcabanks_free(xchg(&mca_allbanks, all));
This function is thread safe and does below:
   1. Atomically exchanges the value of "mca_allbanks" with "all"
   2. Returns the old value that was previously in "mca_allbanks"
So, when we will call mcabanks_free , that will free the memory.

The problem is that mcabanks_set(i, mca_allbanks) function is updating
mca_allbanks which will be freed via mcabanks_free later. This means
new mca_allbanks instance("all") will never get chance to update
it's bank_map.

Due to this when we will collect log from mcheck_mca_logout function ,
the condition "if ( !mcabanks_test(i, bankmask) )" will always fails
and MCA logs will not be collected for any bank.

The fix is to solve this problem.

Fixes: 560cf418c8 ("x86/mcheck: allow varying bank counts per CPU")
Signed-off-by: Soham Dandapat <soham.dandapat@amd.com>
Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 532988dccc
master date: 2025-09-09 08:18:11 +0200

xen/arch/x86/cpu/mcheck/mce.c

@@ -663,7 +663,7 @@ static int mca_cap_init(void)
         if ( !all )
             return -ENOMEM;
         for ( i = 0; i < nr; i++ )
-            mcabanks_set(i, mca_allbanks);
+            mcabanks_set(i, all);
         mcabanks_free(xchg(&mca_allbanks, all));
     }