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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 30 Sep 2005, at 17:34, Hollis Blanchard wrote:

> > Cross-architecture 'compatibility' (same binary layout) is not 
> > currently an aim for the Xen-public interfaces, and I don't expect it 
> > to become so. If we went down that road we'd have to stipulate things 
> > like endianess, which I think we can all agree is not the way to go.
> > I certainly don't want to wholesale restructure our interfaces just 
> > to fortuitously make things match up for 32- and 64-bit ppc (which is 
> > what you are actually arguing for, in the guise of more general 
> > cross-architecture compatibility).
> Yes, I'm not trying to be sneaky about this: this is a problem for 
> ppc64. It could also be a problem for x86-64, except that x86-32 is so 
> limited compared to x86-64 that apparently nobody wants to do this.
> 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...
There's the rub: we don't expect to ever want to provide 32-bit x86 ABI 
compatibility on 64-bit x86 Xen. We will not be supporting 32-bit 
paravirtualised guests on 64-bit x86 Xen, and we've taken the approach 
of requiring separate 32- and 64-bit toolsets (which isn't too painful 
on x86 since full-fledged 64-bit filesystems are quite easy to come 
by).
Really this sounds to me like this is only going to be a problem for 
ppc. That given, knocking up a translation script to import the 
interfaces into your 32-bit toolchain does seem worthy of 
consideration. It wouldn't allow you to use the same tool binaries on 
both 32- and 64-bit ppc Xen, but I don't know how much you actually 
care about that...
 -- Keir


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