Re: [Xen-devel] [PATCH v3 ]libxl: allow to set more than 31 vcpus

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2012-06-01 at 09:44 +0100, Dario Faggioli wrote:
> On Fri, 2012-06-01 at 07:18 +0000, Zhang, Yang Z wrote: 
> > > > diff -r 3b0eed731020 tools/libxl/xl_cmdimpl.c
> > > > --- a/tools/libxl/xl_cmdimpl.c  Fri Jun 01 09:27:17 2012 +0800
> > > > +++ b/tools/libxl/xl_cmdimpl.c  Fri Jun 01 10:34:13 2012 +0800
> > > > @@ -650,7 +650,14 @@ static void parse_config_data(const char
> > > >
> > > >      if (!xlu_cfg_get_long (config, "vcpus", &l, 0)) {
> > > >          b_info->max_vcpus = l;
> > > > -        b_info->cur_vcpus = (1 << l) - 1;
> > > > +
> > > > +        if (libxl_cpumap_alloc(ctx, &b_info->avail_vcpus, l)) {
> > > > +            fprintf(stderr, "Unable to allocate cpumap\n");
> > > > +            exit(1);
> > > > +        }
> > > >
> > > ... Do you mind explaining me what would have happened here without your
> > > previous patch, i.e., by just using the existing libxl_cpumap_alloc ?
> > >
> > > I might be wrong, but I was wondering whether it is worth changing the
> > > interface like that for just this single case which saves, what, 1 to 3
> > > bytes per domain?
> > > 
> > 
> > It's ok to use existing libxl_cpumap_alloc(). But in my case, there is no 
> > need to use the existing interface.
> >
> Ok.
> 
> > And, in future, there are some cases may not need to allocate max size 
> > cpumap too
> > So it's better to extend the current interface.
> > 
> Well, maybe... Who knows what future reserves ?!? :-D
> 
> Anyway, although I see your point, I really really dislike the new
> parameter in libxl_cpumap_alloc(),

What about it do you dislike? The special meaning of 0 or its existence
at all?

> but of course it is not something up
> to me to decide, neither it is something I'd loose some sleep for. :-P

You could give vcpus > pcpus (dumb, but e.g. for debugging) and in that
case the existing libxl_cpumap_alloc behaviour (which sizes based on the
# of phys cpus) is incorrect. I suggested that rather than having
libxl_cpumap_alloc_size() we just combine this with the existing fn with
a new parameter.

Ian.


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