[PATCH v1 7/7] spec/aarch64: Force strict alignment for ZynqMP

Kinsey Moore kinsey.moore at oarcorp.com
Fri Mar 5 13:12:27 UTC 2021

Hi Sebastian,
For AArch64, that would be SCTLR_EL1.A (bit 1). Even with that alignment
checking disabled, I still see data aborts on misaligned accesses. The MMU is
disabled during operation, so that wouldn't be the issue, either. From reading
the spec on that bit, it would seem that AArch64 is accepting of misaligned
writes by default, but that's not the behavior I've seen with the two ZynqMP
boards I have on hand.

There is also SCTLR_EL1.nAA which toggles allowance of accesses spanning
16-byte boundaries, but the feature controlling whether that bit functions is
only mandatory from ARMv8.4 onward and is optional from ARMv8.2. Even
so, the nAA bit only affects a small subset of load and store instructions.

The section on unaligned accesses in the manual only references these two
bits and the LSE2 feature that controls whether the nAA bit is functional.


-----Original Message-----
From: Sebastian Huber <sebastian.huber at embedded-brains.de> 
Sent: Thursday, March 4, 2021 23:54
To: Kinsey Moore <kinsey.moore at oarcorp.com>; devel at rtems.org
Subject: Re: [PATCH v1 7/7] spec/aarch64: Force strict alignment for ZynqMP

On 04/03/2021 23:15, Kinsey Moore wrote:

> Real hardware running AArch64 does not appreciate accesses misaligned
> relative to the data size. This prevents generation of misaligned writes
> which would throw exceptions.

The patch set is fine independent of the following comment.

To me this problem with misaligned access looks like an MMU/system 
configuration issue. For example in AArch32, you have to enable 
misaligned access in SCTL[A].

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