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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Jul 16, 2015 at 10:57:24AM +0200, Dario Faggioli wrote:
> [Adding Juergen]              
> 
> 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.
> 
> > 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?
> 

See below.

> Maybe, depending on the error, we can cleanup the whole array? Is it
> this that we are after?
> 
> > 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?
> > 
> Sorry, what you mean by 'interleave pool ids'?
> 

For example, pools have ids 0 1 4 6. In this case when should we stop
querying hypervisor about cpu pools? Current model will stop at id 2.

So my first question is if the example I mention is valid. If not, we
can use the same trick as we do now, otherwise we need some way to get
the number of cpu pools and the upper bound of ids.

Wei.

> Dario
> -- 
> <<This happens because I choose it to happen!>> (Raistlin Majere)
> -----------------------------------------------------------------
> Dario Faggioli, Ph.D, http://about.me/dario.faggioli
> Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)



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