Re: [Xen-devel] [PATCH 1/5] xen: improve changes to xen_add_to_physmap

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 6 Aug 2012, Jan Beulich wrote:
> >>> On 06.08.12 at 17:43, Stefano Stabellini 
> >>> <stefano.stabellini@xxxxxxxxxxxxx>
> wrote:
> > On Mon, 6 Aug 2012, Jan Beulich wrote:
> >> >>> On 06.08.12 at 16:12, Stefano Stabellini 
> >> >>> <stefano.stabellini@xxxxxxxxxxxxx>
> >> wrote:
> >> > This is an incremental patch on top of
> >> > c0bc926083b5987a3e9944eec2c12ad0580100e2: in order to retain binary
> >> > compatibility, it is better to introduce foreign_domid as part of a
> >> > union containing both size and foreign_domid.
> >> > 
> >> > Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> >> > ---
> >> >  xen/include/public/memory.h |   11 +++++++----
> >> >  1 files changed, 7 insertions(+), 4 deletions(-)
> >> > 
> >> > diff --git a/xen/include/public/memory.h b/xen/include/public/memory.h
> >> > index b2adfbe..b0af2fd 100644
> >> > --- a/xen/include/public/memory.h
> >> > +++ b/xen/include/public/memory.h
> >> > @@ -208,8 +208,12 @@ struct xen_add_to_physmap {
> >> >      /* Which domain to change the mapping for. */
> >> >      domid_t domid;
> >> >  
> >> > -    /* Number of pages to go through for gmfn_range */
> >> > -    uint16_t    size;
> >> > +    union {
> >> > +        /* Number of pages to go through for gmfn_range */
> >> > +        uint16_t    size;
> >> > +        /* IFF gmfn_foreign */
> >> > +        domid_t foreign_domid;
> >> > +    };
> >> 
> >> But you're clear that this isn't standard C, and hence can't go
> >> in this way?
> >> 
> > 
> > Why? It is c11 if I am not mistaken.
> 
> Yes. But the common baseline is C89.

Do we need to keep it C89?
If I am not mistaken anonymous unions have been in GCC for more than 10
years now.

If we do want to keep it C89, considering that size was introduced only
recently, do you think that it would be OK for me to change the
interface and just add size to a union?
Like this:

union {
    /* Number of pages to go through for gmfn_range */
    uint16_t    size;
    /* IFF gmfn_foreign */
    domid_t foreign_domid;
} u;

Also CC'ing Jean.

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