Re: [Xen-devel] [PATCH v3 for Xen 4.6 3/4] libxl: enable per-VCPU parameter settings for RTDS scheduler

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Wed, 2015-07-01 at 09:48 +0100, Ian Campbell wrote:
> On Tue, 2015-06-30 at 17:54 -0700, Meng Xu wrote:
> > 2015-06-30 9:19 GMT-07:00 Ian Campbell <ian.campbell@xxxxxxxxxx>:
> > > Note that this field is not the same as the others in this struct, it is
> > > in effect part of the "key" while the others are the "values".
> > >
> > 
> > vcpuid in the libxl is not the key but the value.
> > 
> > When it is passed into hypervisor, vcpuid acts as the key to identify
> > which VCPU. vcpuid should be be negative when it is passed to
> > hypervisor.
> > 
> > Do you have any concerns about assigning the initial value of vcpuid as -1?
> 
> libxl needs to do _something_ if it is passed a vcpuid=-1. What is that
> something?
> 
Exactly.

Meng, Chong, maybe what you are missing from Ian's point is the fact
that libxl is a library on top of which to build more advanced
toolstack, it is not just something that the xl command line tool links
to.

Therefore, it has to work even when it is not xl that makes the API
calls. Actually, I better say that --not only it has to work-- it has to
be easy and comfortable to use from high level components different than
xl.

The outcome of this reasoning is that, you can't just assume that a
default value does not matter much, because every code path in _xl_
overrides it. Everything that is in libxl must make sense in libxl
per-se, even without bringing xl into the picture.

So, in this case, you need to think whether it is convenient
_in_general_, not only wrt xl, to have -1 as default value for vcpuid
and, if yes, what happens if such default is not overridden and libxl
gets to deal with it!

Hope this helps... If this was not the issue, sorry for the noise. :-)

Regards,
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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