From: Russell King - ARM Linux
> Sent: 01 October 2016 20:52
> On Fri, Sep 30, 2016 at 07:16:12AM -0700, Eric Nelson wrote:
> > On ARM, the CPU can't handle misaligned memory cycles without
> > taking an alignment fault and NET_IP_ALIGN is set to 2.
>
> Let's get this right... With Linux on MMU parts:
>
> On ARMv6+, unaligned memory cycles using the LDR, LDRH and corresponding
> store instructions are handled in hardware without any exception being
> raised.
>
> On pre-ARMv6, such instructions raise an alignment exception, and we fix
> up the load/store manually.
I'm not sure that is a good idea but...
> Where things behave the same is with the LDM (load multiple) and STM
> (store multiple) instructions. Hardware does not fix these up if they
> are unaligned: it is expected that the base address will always be
> aligned to a 32-bit word.
>
> For some reason, the compiler guys have decided it's okay to use these
> instructions as an optimisation, and I see no way to disable this
> behaviour.
...
> The issue with the networking layer is that it passes around structure
> pointers which may not be "naturally aligned" - technically it goes
> against the C standard specs. However, you'll find it hard to argue
> against this, so we have to accept that the networking people expect
> it to work.
I think it 'only' casts misaligned pointers to structure on systems
where unaligned accesses are allowed.
It is almost impossible to do a 'realignment copy' on (for example) sparc.
> The optimisation that the C compiler uses (using LDM to access multiple
> 32-bit consecutive words) is legal and efficient when the structure
> pointers are aligned as it expects, but that all breaks if the pointer
> is not so aligned. So, raising it as a bug against the C compiler isn't
> going to work either.
>
> What may work is to raise a feature request with compiler people to have
> a mechanism to disable the LDM/STM optimisation for code where we know
> that pointers may not be naturally aligned.
What happens is the relevant structures are marked 'packed'?
If the compiler still generates LDM/STM that is a bug.
Assuming the compiler is targeting v6+ it shouldn't generate byte accesses.
David
