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

[Jeremy Katz <katzj@xxxxxxxxxx>]

On Fri, 2005-09-30 at 17:42 +0100, Keir Fraser wrote:
> 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, 

... which I've previously said and continue to think is going to be
something that will eventually be regretted.  People are going to want
to continue to run in a 32bit OS for legacy reasons for quite a while
(even with the compatibility you get on x86_64).  Making it so they
can't do mix and match of 32 and 64 bit guests on a single host is going
to significantly reduce the utility of Xen.

Jeremy


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