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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 27 Feb 2015, Juergen Gross wrote:
> On 02/26/2015 06:42 PM, Stefano Stabellini wrote:
> > 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.
> 
> Hmm, after checking the code I'm not convinced:
> 
> - HVMOP_pagetable_dying is obsolete on modern hardware supporting
>   EPT/HAP

That might be true, but what about older hardware?
Even on modern hardware a few workloads still run faster on shadow.
But if HVMOP_pagetable_dying is the only reason to keep PARAVIRT for HVM
guests, then I agree with you that we should remove it.


> - PV IPIs are not needed on single-vcpu guests
>
> - PARAVIRT_CLOCK doesn't need PARAVIRT (in fact the SUSEs kernel configs
>   for all x86_64 kernels have CONFIG_PARAVIRT_CLOCK=y)
> 
> So I think we really should enable building Xen frontends without
> PARAVIRT, implying at least no XEN_PV and no XEN_PVH.
> 
> I'll have a try setting up patches.
 
If we are doing this as a performance improvement, I would like to see a
couple of benchmarks (kernbench, hackbench) to show that on a
single-vcpu guest and multi-vcpu guest (let's say 4 vcpus) disabling
PARAVIRT leads to better performance on Xen on EPT hardware.

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