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

["Luis R. Rodriguez" <mcgrof@xxxxxxxx>]

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.

  Luis

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