MMU Off / Strict Alignment

Christopher Covington
Tue Dec 17 00:54:00 GMT 2013


On 11/20/2013 03:45 PM, Matthew Gretton-Dann wrote:
> On 20 November 2013 17:57, Christopher Covington <> wrote:
>> Hi,
>> We've noticed an issue trying to use the Linaro AArch64 binary bare metal
>> toolchain release with the MMU turned off for some low-level tests.
>> Anytime puts, sprintf, etc. gets called, a reent structure gets created with
>> references to STDIN, STDOUT, STDERR FILE types. A member in the __sFile
>> struct, _mbstate, is an 8 byte struct, but is not aligned on an 8 byte
>> boundary. This means that when memset (or a similar function) gets called on
>> this struct, and doesn't operate one byte at a time, a data alignment fault
>> will be generated when operating out of device memory, such as on a system
>> where the MMU has not yet been turned on yet.

We believe to have narrowed down the issue to the AArch64 optimized
memcpy/memset implementations that assume unaligned accesses will not fault.
While the current AArch64 libgloss startup code turns the MMU on so such
accesses will succeed, I don't think turning on the MMU should be required of
all startup code. Would it be possible to modify these routines to make only
size-aligned accesses without degrading performance? If a single
implementation can't make everyone happy, should the ifdefs around them
perhaps be expanded to include something about requiring the MMU to be on?


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