Re: Process to push changes to include/linux/types.h

[Andreas Gruenbacher]

On Friday 15 October 2010 11:01:34 Andrew Morton wrote:
> On Fri, 15 Oct 2010 10:22:49 +0200 Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> 
> > Jan Engelhardt <jengelh@xxxxxxxxxx> writes:
> > 
> > > I would be interested in knowing whether you - in whichever subsystems
> > > you happen to be active - would even need aligned_u64. Right now,
> > > the only users seem to be PPP and scsi_tgt besides Netfilter.
> > 
> > Using aligned_u64 is good practice to avoid problems with the 
> > 32bit/64bit compat layer. I would recommend it to anyone 
> > adding a new user space interface passing a 64bit value.
> > 
> 
> What "problems" does it prevent when used for this?

64-bit values align to 4-byte boundaries on some 32-bit architectures like x86 
and to 8-byte boundaries on 64-bit architetures.  The new __aligned_64 type 
enforces 8-byte alignment and so structs containing __aligned_64 values have 
the same alignment on 32-bit and 64-bit architectures.  No conversions are 
necessary between 32-bit user-space and a 64-bit kernel.

Andreas
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