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Add a note about mixed-bitness applications
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From the Linux kernel's point of view, applications differ by values set during exec of application such as <code>mmap_base</code> or thread info flags <code>TIF_ADDR32</code>/<code>TIF_IA32</code>/<code>TIF_X32</code>.
From the Linux kernel's point of view, applications differ by values set during exec of application such as <code>mmap_base</code> or thread info flags <code>TIF_ADDR32</code>/<code>TIF_IA32</code>/<code>TIF_X32</code>.
Both native and compat applications can do 32 or 64-bit syscalls.
Both native and compat applications can do 32 or 64-bit syscalls.
+
+== Mixed-bitness applications ==
+
+That's entirely possible with current kernel ABI to create mixed-bitness applications, which may be ''very'' entangled.
+For example, one could set ''both'' 32-bit and 64-bit robust futex list pointers.
+Or one can create multi-threaded application where some threads are executing 32-bit code, some 64-bit code.
+
+If we ever meet application of such mixed-bitness kind, the support may be added to CRIU quite easily, but it should be done under some compile-time config as it'll add more syscalls to usual C/R where they aren't needed.
+
+At this moment there is no plans to add such support and it's quite unlikely that we'll find such application in real world (non-syntetic test).
== Approaches to C/R compatible applications ==
== Approaches to C/R compatible applications ==