Re: [Xen-devel] 32/64-bit hypercall interface

[Hollis Blanchard <hollisb@xxxxxxxxxx>]

On Sep 30, 2005, at 11:44 AM, David wrote:
> > I think the Linux kernel folks learned the "unsigned long" lesson too 
> > late, and now there is quite a lot of fixup code to convert 32-bit 
> > userspace structs to 64-bit kernel structs (have a look at 
> > linux/fs/compat_ioctl.c and compat.c). It seems a shame to repeat the 
> > same mistakes in Xen...
> Quick question....  What is this "'unsigned long' lesson" you are 
> refering to?
"long" has the unfortunate tendency to change size between 32- and 
64-bit compiles, like a pointer. So if you put that in a structure, the 
structure's layout will change between 32- and 64-bit builds. When you 
make that structure part of the userland/kernel ABI, you will have 
problems on architectures that can run both 32- and 64-bit binaries, 
which include PowerPC, MIPS, x86-64, Sparc, and possibly S390 and even 
IA64.
--
Hollis Blanchard
IBM Linux Technology Center


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