Re: [Xen-devel] [RFC][PATCH 01/13] tools: introduce some new parameters to set rdm policy

["Chen, Tiejun" <tiejun.chen@xxxxxxxxx>]

On 2015/5/20 16:36, Wei Liu wrote:
> On Wed, May 20, 2015 at 01:27:56PM +0800, Chen, Tiejun wrote:
> [...]
> > > > We have this definition,
> > > >
> > > > +libxl_rdm_reserve_type = Enumeration("rdm_reserve_type", [
> > > > +    (0, "none"),
> > > > +    (1, "host"),
> > > > +    ])
> > > >
> > > > If we set 'type=none', this means we would do nothing actually since we
> > > > don't expose any rdms to guest. This behavior will ensue we go back the
> > > > existing scenario without our patch.
> > > But this only works with global configuration and individual
> > > configuration in PCI spec trumps this, right?
> > You're right.
> >
> > > Think about how an old configuration migrated to newer version of Xen
> > > should work. For example, I don't have rdm= but have pci=['xxxx']. Do we
> > > need to make sure this still work? I guess the answer is if it already
> > Definitely.
> >
> > > works before RDM it should continue to work as there is really no
> > > conflict before. In this case whether  we enable RDM or not doesn't make
> > > a difference to a guest that's already working before. Am I right?
> You haven't answered this question...  I'm trying to determine what
> should be a sensible default value.
I think I should say 'no' here. RDM (RMRR) already exists and its also 
being enabled before I'm trying to introduce this series of patch. But 
we have some legacy or potential problems to really work well with RMRR.
> If the answer to that question is "yes", then we should enable RDM by
> default, because it does no harm to guests that are already working and
> fix problem for the guests that are not working; if the answer is "no"
> or "not sure", we should use "none". Don't worry, we can change the
> default value later if necessary.
So we're going to the latter :)

> Using "none" as default leaves us on the safe side but it would make it
> less nicer to use Xen. But well, safety comes first.
Actually RMRR doesn't matter in most cases unless you're trying to do 
pass through with a device which owns RMRR and also really use RMRR 
indeed. So from my point of view, I agree "NONE" should be default since 
we really should make sure we have a way to approach our original 
behavior in accordance with your concern.
Thanks
Tiejun

> Wei.
>
> > I think we can set the default 'type' to NONE,
> >
> > libxl__rdm_setdefault()
> > {
> >      b_info->rdm.type = LIBXL_RDM_RESERVE_TYPE_NONE;
> >
> > and then,
> >
> > libxl__domain_device_construct_rdm()
> > {
> >      ...
> >      /* Might not expose rdm. */
> >      if (type == LIBXL_RDM_RESERVE_TYPE_NONE)
> >         return 0;
> >
> > This means we don't expose any rdm so we would never concern any policy
> > anymore.
> >
> >
> > Thanks
> > Tiejun
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