Re: [Xen-devel] [PATCH v3 10/10] libxl: fix caller of libxl_cpupool functions

[Juergen Gross <jgross@xxxxxxxx>]

On 07/16/2015 10:57 AM, Dario Faggioli wrote:
> [Adding Juergen]
Near miss. You took my old mail address. :-)

> On Wed, 2015-07-15 at 18:16 +0100, Wei Liu wrote:
> > On Tue, Jul 14, 2015 at 06:33:22PM +0100, Ian Jackson wrote:
> > > Wei Liu writes ("[PATCH v3 10/10] libxl: fix caller of libxl_cpupool 
> > > functions"):
> > > > --- a/tools/libxl/libxl.c
> > > > +++ b/tools/libxl/libxl.c
> > > > @@ -771,8 +771,11 @@ libxl_cpupoolinfo * libxl_list_cpupool(libxl_ctx *ctx, int 
> > > > *nb_pool_out)
> > > >
> > > >       poolid = 0;
> > > >       for (i = 0;; i++) {
> > > > -        if (cpupool_info(gc, &info, poolid, false))
> > > > +        libxl_cpupoolinfo_init(&info);
> > > > +        if (cpupool_info(gc, &info, poolid, false)) {
> > > > +            libxl_cpupoolinfo_dispose(&info);
> > > >               break;
> > > > +        }
> > > I'm not convinced by this change.
> > >
> > > I think that this function has broken error handling: if cpupool_info
> > > fails, it simply ignores the error.
> > I think the semantics of this function is to get back as many cpupool
> > info as it can.
> Yes, I also think this is the case.
Indeed.

> > Unfortunately the failing of cpupool_info can either mean the cpupool id
> > does not exist or other internal errors.  So not cleaning up is the
> > right thing to do (speaking from semantics point of view).
> Indeed.
>
> > I think I need to overhaul cpupool_info a bit if we want to make this
> > API better.
> Well, perhaps having cpupool_info() treating specially the situation
> where the pool ID is not there may help... However, what would you do
> once you have this additional piece of information available?
>
> Maybe, depending on the error, we can cleanup the whole array? Is it
> this that we are after?
I think the best would be:

Modify cpupool_info() to return either success, internal error, or no
cpupool found.

In case of internal error libxl_list_cpupool() should clean up and
return NULL.

> > I'm not sure if it is possible to interleave pool ids. If so, we need a
> > way to get back the exact number of cpupools. Dario?
libxl_list_cpupool() does that (2nd parameter).

> >
> Sorry, what you mean by 'interleave pool ids'?
Not sure if this is an answer to interleaving of pool ids, but it is
possible to specify the pool id when creating a new cpupool at the
libxc interface. Even in case this is not used, pool ids can easily
be sparse after having deleted a cpupool.


Juergen


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