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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Jun 25, 2018 at 05:18:31PM +0100, Julien Grall wrote:
> Hi Wei,
> 
> On 25/06/18 14:19, Wei Liu wrote:
> > On Mon, Jun 25, 2018 at 02:08:14PM +0100, Julien Grall wrote:
> > > Currently, the toolstack is considering Arm guest always PV. However,
> > > they are very similar to PVH because HW virtualization extension are used
> > > and QEMU is not started. So switch Arm guest type to PVH.
> > > 
> > > Furthermore, the default type for Arm in xl will now be PVH to allow
> > > smooth transition for user.
> > > 
> > > Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> > > 
> > > ---
> > > 
> > > This was discussed at Xen Summit and also in various thread on
> > > xen-devel. The latest one was when Andrew sent a patch to deny guest 
> > > creation
> > > on Arm with XEN_DOMCTL_CDF_hap unset.
> > > 
> > > I suspect we first implemented Arm guest as PV in libxl because PVH was
> > > non-existent and the type was easier to avoid spawning QEMU. Note that
> > > Linux and Xen are already considering Arm guest as PVH.
> > > 
> > > This patch is denying the creation of PV guest on Arm. This will have an
> > > impact on any toolstack selecting PV for Arm (e.g libvirt) or anyone
> > > using 'type="pv"' in libxl configuration file.
> > > 
> > > I am not entirely sure whether this is the best solution, any opinions?
> > > ---
> > >   docs/man/xl.cfg.pod.5.in |  3 ++-
> > >   tools/libxl/libxl_arm.c  | 10 +++++++++-
> > >   tools/xl/xl_parse.c      |  4 ++++
> > >   3 files changed, 15 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/docs/man/xl.cfg.pod.5.in b/docs/man/xl.cfg.pod.5.in
> > > index 47d88243b1..2c2af01515 100644
> > > --- a/docs/man/xl.cfg.pod.5.in
> > > +++ b/docs/man/xl.cfg.pod.5.in
> > > @@ -86,7 +86,8 @@ guest operating systems. This is the default.
> > >   Specifies that this is to be an PVH domain. That is a lightweight 
> > > HVM-like
> > >   guest without a device model and without many of the emulated devices
> > > -available to HVM guests. Note that this mode requires a PVH aware kernel.
> > > +available to HVM guests. Note that this mode requires a PVH aware kernel 
> > > on
> > > +x86.
> > 
> > Have we ever advertised on ARM which guest type users should set? Are
> > they aware of the fact that it is PV in toolstack and HVM in Xen?
> 
> We always advertised "ARM guest" and not a specific type of guest. So most
> of the users of xl will likely not specified the type of guest.
> 
> > 
> > If users are oblivious to the underlying implementation, I think we can
> > skip telling them PVH in ARM world.
> PVH is quite confusing for Arm. We only used the term to make a comparison
> with x86. This is the closest to Arm guest but I am not too sure whether
> they would be the exactly the same.
> 
> At the moment, I would rather avoid to say 'PVH' for the end user and just
> let the different toolstack to set the type to PVH.
> 
> Another solution is for libxl to force the guest type to PVH on Arm. I am
> not sure what would be the impact here.

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).

Iff ARM even gains more guest types the option can be added and the
guest types can be set to meaningful ARM guest types.

In any case, this requires changes to xl.cfg man page.

Roger.

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