ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

Note
Virtual Memory System Architecture (VMSA)
Another instance of the synchronization problem occurs if a branch is encountered between changing the
ASID and performing the synchronization. In this case the value in the branch predictor might be associated
with the incorrect ASID. This possibility can be addressed by any of these approaches, but might be
addressed by avoiding such branches.
Example B3-2 Using a reserved ASID to synchronize ASID and TTBR changes
In this approach, a particular ASID value is reserved for use by the operating system, and is used only for
the synchronization of the ASID and Translation Table Base Register. This example uses the value of 0 for
this purpose, but any value could be used.
This approach can be used only when the size of the mapping for any given virtual address is the same in
the old and new translation tables.
The following sequence is followed, and must be executed from memory marked as being global:
Change ASID to 0
ISB
Change Translation Table Base Register
ISB
Change ASID to new value
This approach ensures that any non-global pages prefetched at a time when it is uncertain whether the old
or new translation tables are being accessed are associated with the unused ASID value of 0. Since the ASID
value of 0 is not used for any normal operations these entries cannot cause corruption of execution.
Example B3-3 Using translation tables that contain only global mappings
when changing the ASID
A second approach involves switching the translation tables to a set of translation tables that only contain
global mappings while switching the ASID.
The following sequence is followed, and must be executed from memory marked as being global:
Change Translation Table Base Register to the global-only mappings
ISB
Change ASID to new value
ISB
Change Translation Table Base Register to new value
This approach ensures that no non-global pages can be prefetched at a time when it is uncertain whether the
old or new ASID value will be used.
ARM DDI 0406B Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. B3-61