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

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- HVMOP_pagetable_dying is obsolete on modern hardware supporting

- 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.


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