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Re: [Xen-devel] RFC: xen config changes v4

On Thu, 26 Feb 2015, David Vrabel wrote:
> On 26/02/15 04:59, Juergen Gross wrote:
> > 
> > So we are again in the situation that pv-drivers always imply the pvops
> > kernel (PARAVIRT selected). I started the whole Kconfig rework to
> > eliminate this dependency.
> Yes.  Can you produce a series that just addresses this one issue.
> In the absence of any concrete requirement for this big Kconfig reorg I
> I don't think it is helpful.

I clearly missed some context as I didn't realize that this was the
intended goal. Why do we want this? Please explain as it won't come
for free.

We have a few PV interfaces for HVM guests that need PARAVIRT in Linux
in order to be used, for example pv_time_ops and HVMOP_pagetable_dying.
They are critical performance improvements and from the interface
perspective, small enough that doesn't make much sense having a separate
KConfig option for them.

In order to reach the goal above we necessarily need to introduce a
differentiation in terms of PV on HVM guests in Linux:

1) basic guests with PV network, disk, etc but no PV timers, no
   HVMOP_pagetable_dying, no PV IPIs
2) full PV on HVM guests that have PV network, disk, timers,
   HVMOP_pagetable_dying, PV IPIs and anything else that makes sense.

2) is much faster than 1) on Xen and 2) is only a tiny bit slower than
1) on native x86

From Xen perspective and from code maintenance perspective I don't think
it makes sense to have the separation, actually it would make things
slower and harder to maintain.

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