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Re: [Xen-devel] [PATCH RFC] tools/libxl: Switch Arm guest type to PVH

On Mon, Jul 02, 2018 at 10:18:17AM -0700, Stefano Stabellini wrote:
> On Mon, 2 Jul 2018, Julien Grall wrote:
> > On 06/26/2018 07:49 AM, Roger Pau Monné wrote:
> > > On Mon, Jun 25, 2018 at 05:39:12PM +0100, Ian Jackson wrote:
> > > > Roger Pau Monné writes ("Re: [PATCH RFC] tools/libxl: Switch Arm guest
> > > > type to PVH"):
> > > > > IMO I would remove the 'type' option from xl.cfg (so that it's
> > > > > basically ignored) in the ARM case and force it internally to PVH (if
> > > > > that's the best route for current ARM guests).
> > > > 
> > > > What about libvirt users ?  I haven't seen what a libvirt Xen ARM
> > > > guest config looks like but we need to meak sure that existing guests
> > > > don't break.
> > > 
> > > For livbirt (or users of libxl library) we could force the type to
> > > pvh, regardless of the value set by the client, but I guess that would
> > > make adding types later on quite complicated.
> > 
> > I am fairly confident we will never have PV guest on Arm. So one solution
> > would be to alias PV to PVH for Arm. This still give us the liberty to add
> > more guest type in the future.
> > 
> > Any opinions?
> Roger, what is the plan for x86? Wasn't there an idea to silently and
> transparently "upgrade" PV guests to PVH when possible (when hardware
> support is available)?

This could only be done for xl config files that don't specify a
'type' option IMO, for libvirt and other users of libxl I don't think
this is feasible.

> If that is the case, basically we could do the same for ARM. We could
> have an hardware features check, that would always return true on ARM
> because without virtualization extensions Xen cannot even boot, then
> upgrade PV to PVH. On x86 the upgrade would only happen when the
> required features are present.

It's slightly more convoluted on x86 because apart from the hardware
features you also need to parse the kernel in order to figure out if
it has the PVH entry point before setting the type to PVH.


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