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Re: MMU Off / Strict Alignment
- From: Christopher Covington <cov at codeaurora dot org>
- To: newlib at sourceware dot org, Marcus Shawcroft <marcus dot shawcroft at linaro dot org>
- Cc: Matthew Gretton-Dann <matthew dot gretton-dann at linaro dot org>, "linaro-toolchain at lists dot linaro dot org" <linaro-toolchain at lists dot linaro dot org>
- Date: Mon, 16 Dec 2013 12:54:34 -0500
- Subject: Re: MMU Off / Strict Alignment
- Authentication-results: sourceware.org; auth=none
- References: <528CF7F1 dot 5050001 at codeaurora dot org> <CADSXKXqJgD3cq594+NeRk9=QHA1DKh3o7aPjsVYOx5OqT1Y6pw at mail dot gmail dot com>
Hi,
On 11/20/2013 03:45 PM, Matthew Gretton-Dann wrote:
> On 20 November 2013 17:57, Christopher Covington <cov@codeaurora.org> 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?
Thanks,
Christopher
--
Employee of Qualcomm Innovation Center, Inc.
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