Re: [Minios-devel] Single codebase for para-virtualized guests on x86 and ARM?

[Julien Grall <julien.grall@xxxxxxx>]

On 05/04/18 09:38, Ajay Garg wrote:
> Hi Julien.
>
> Thanks for the reply.
>
> On Thu, Apr 5, 2018 at 1:21 PM, Julien Grall <julien.grall@xxxxxxx> wrote:
> > (+ Stefano and Andre)
> >
> > On 04/05/2018 04:35 AM, Ajay Garg wrote:
> > > Hi Julien.
> > >
> > > Sorry for restarting this thread, but I was just wondering whether PV
> > > on ARM might not provide the following benefits :
> > >
> > >      * Low overheads as compared to HVM.
> >
> > I don't buy this argument. The overhead is now quite high because with a PV
> > solution as you would need to mitigate meltdown.
> >
> > Furthermore, this require a lot of changes in your guests and I don't see
> > any OS community accepting such burden when virtualization extension is
> > getting quite widespread on Arm.
> >
> > >      * Also support systems/procedures with no hardware-virtualization
> > > support.
> >
> > There was an attempt to get PV support for Arm before the virtualization
> > support was added ([1]). But this is dead.
> >
> > Do you have any platform in mind you would want to run Xen with no
> > virtualization extension?
> Ideally, we are targetting low-end arm systems, where
> hardware-virtualization is the one thing that we might have to
> compromise on.
> Beaglebones are an example.
They are low-end platform with hardware-virtualization (e.g pine64).
So why that platform in particular? Is it because you have existing 
hardware?
To be clear, I think compromising virtualization extension is a pretty 
bad idea. The burden on software will really be huge compare to the 
current solution on Xen Arm.
Cheers,

> > > Of course, the benefits of PV need to be really high, to justify the
> > > efforts needed to make the required code-changes for PV-support :P
> >
> > HW virtualization support provides good security and performance with
> > limited changes in the guest. Even, x86 community has been discussing to
> > deprecate PV and move towards PVH (very similar to Arm guest).
> >
> > Cheers,
> >
> > [1] https://wiki.xenproject.org/wiki/Archived/Xen_ARM_(PV)
> >
> > --
> > Julien Grall
--
Julien Grall

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