Re: block layer sg, bsg

[Daniel Phillips]

Hi Dave,

On Wednesday 04 August 2004 23:58, David S. Miller wrote:
> On Wed, 04 Aug 2004 16:51:13 -0700 (PDT)
>
> YOSHIFUJI Hideaki / $B5HF#1QL@(B <yoshfuji@xxxxxxxxxxxxxx> wrote:
> > > Well, 32bit ipsec on x86-64/ia64 is a NOP because of that.
> >
> > Hmm, I don't get the point.
> > What part (or which structore) is broken?
>
> On x86-64 and ia64 they have a large issue because the ia32
> emulation layer has to handle the fact that "long long" types
> do not require 8-byte alignment, whereas 64-bit natively
> they do.

I'm making heavy use of struct read/writes in the cluster snapshot block 
device, so I need to get this sorted out.

Somewhere I got the idea that if a structure is declared with attribute 
PACKED, gcc will generate alignment-independent code (e.g., access each field 
byte by byte) on alignment-restricted architectures.  So if what I imagine 
about gcc is true, what issues remain?  These structs have to be declared 
packed anyway and with fixed field sizes, or the layout will vary across 
architectures.

Regards,

Daniel
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