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

[Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>]

On Wed, 28 May 2014 10:42:37 +0100
Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:

> 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>
> > >>>>>> ---
...
> > >> 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.

Correct, thats how I visualized it would be some day when the pending
work including performance, is all done.

> 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.

Concur again. That would somewhat enforce/encourage problems to be 
reported/caught IMO.

thanks,
Mukesh


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