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Re: [Xen-devel] [PATCH] x86: Expose hypervisor's PVH support via xen_caps

On Fri, 2014-05-23 at 11:53 -0400, Boris Ostrovsky wrote:
> On 05/23/2014 11:35 AM, Roger Pau Monnà wrote:
> > On 23/05/14 17:32, Boris Ostrovsky wrote:
> >> On 05/23/2014 11:20 AM, Andrew Cooper wrote:
> >>> On 23/05/14 16:08, Boris Ostrovsky wrote:
> >>>> On 05/23/2014 11:00 AM, Andrew Cooper wrote:
> >>>>> On 23/05/14 15:55, Boris Ostrovsky wrote:
> >>>>>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> >>>>>> ---
> >>>>>>     xen/arch/x86/setup.c | 5 +++++
> >>>>>>     1 file changed, 5 insertions(+)
> >>>>> If the plan is to try and PVH and HVM back into one mode as far as Xen
> >>>>> is concerned, doesn't this become redundant?
> >>>> Yes, I was thinking about this but we currently don't have (or,
> >>>> rather, I can't think of) a good way to determine whether we can start
> >>>> a PVH guest. We can grep the log but that doesn't feel like a
> >>>> particularly good solution.
> >>>>
> >>>> One option could be to postpone this patch until 4.5 freezes and see
> >>>> whether we indeed followed up on the plan and if we didn't then
> >>>> integrate it.
> >>>>
> >>>> -boris
> >>> My concern here is that if this patch gets accepted, it will have to say
> >>> forever more as the cap strings are a very public API.
> >> Yes, that's true.
> >>
> >> The problem that we have now is that if we have 'pvh=1' in the config
> >> file the guest will fail to start if PVH is not on. Can we, for example,
> >> revert (with a warning) to pure PV if that's the case?
> > Won't this option go away once PVH is stable, so the toolstack can
> > detect if the kernel supports PVH and start the guest in this mode by
> > default? (of course falling back to PV if PVH is not supported).
> Yes, it will, by virtue of hypervisor never having to take this action 
> (reverting to PV) since PVH would be stable and always supported.

This ignores hardware which cannot support pvh and guest kernels which
are lacking the support for it.

> I just don't know whether ignoring pvh=1 directive on systems where PVH 
> is not supported is acceptable. (Your "of course" seems to indicate that 
> you think it is.)

The ideal case would be that you don't say pvh=anything and the
toolstack will automatically use pvh if the h/w and kernel both support
it, otherwise it will use pv.

If you say pvh=1 then the toolstack should unconditionally attempt to
create a pvh guest, and if the hypervisor rejects an attempt to create
such a domain then it should fail, because this is what you have asked
for with pvh=1.


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