Re: [Xen-devel] struct shared_info extensibility (or lack thereof)

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 28 Nov 2005, at 18:01, Rik van Riel wrote:

> It would allow us to grow the maximum supported number of
> VCPUs in the future, without breaking backwards compatibility.
Any extension in the future can easily be made to backwards support old 
OSes, although they will be restricted to 32 VCPUs.
> Of course, it would be useful to then put a few longs worth
> of padding in front of the VCPU array, so we also have expansion
> space in the non-per-VCPU part of shared_info.
Well, I'll leave vcpu_info where it is. We could maybe add some padding 
directly after it, but not sure it's worth it.
> Two or three longs worth of padding per VCPU would be great, too. ;)
Six bytes plus another eight bytes are easily available (padding 
vcpu_info up to 64 bytes).
> Having an interface that can be extended compatibly in the
> future will make everybody's life so much easier.  Improving
> Xen without breaking the already deployed virtual machines.
Sure, but extending MAX_VIRT_CPUS to 64 will require a big change one 
way or another. Either shared_info becomes multi-page, or we explicitly 
put extra vcpu_info's on a separate page. Probably we will do the 
latter if required.
 -- Keir


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel