Re: [PATCH RFC 1/3] xen/save: pass a size paramter to the HVM compat functions

["Roger Pau Monné" <roger.pau@citrix.com>]

El 15/10/15 a les 10.12, Jan Beulich ha escrit:
>>>> On 14.10.15 at 18:24, <roger.pau@citrix.com> wrote:
>> In order to cope with types having multiple compat versions pass a parameter
>> to the fixup function so we can identify which compat version Xen is dealing
>> with.
> 
> Having peeked at patch 2, this won't help once another bit gets added
> to the tail of that structure. Also it doesn't seem logical that the
> previous compat handling got around without being passed the size
> explicitly. I.e. while perhaps more involved, I think the compat
> handling needs to be extended to allow for multiple versions.

Yes, patch #2 needs to be fixed by making fpu_initialised an uint32_t.
Adding versions would be the best option IMHO, but I don't think I have
enough time to do that myself right now.

> Or, since we have this under control going forward, don't even declare
> all the various compat structures in the public header (and only ever
> add to the tail). Then staying with the passing of size probably makes
> sense, but the fixup function then should use offsetof() instead of
> sizeof() (and validate unused tail bits are zero, so they can be used for
> something later on).

Since we use hvm_load_entry_zeroextend I think we can assert that all
new tail bits are going to be 0, but I'm not sure I follow you regarding
the usage of offsetof instead of sizeof. What are we going to compare
with offsetof?

Also, we already have a compat version of hvm_hw_cpu that didn't add the
new field to the end.

Roger.