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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 26 Feb 2015, Luis R. Rodriguez wrote:
> On Thu, Feb 26, 2015 at 11:08:20AM +0000, Stefano Stabellini wrote:
> > 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
> 
> Also don't we shove 2) down hvm guests right now? Even when everything is
> built in I do not see how we opt out for HVM for 1) at run time right now.
>
> If this is true then the question of motivation for this becomes even
> stronger I think.

Yes, indeed there is no way to do 1) at the moment. And for good
reasons, see above.

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