Re: [Xen-devel] HVMLite / PVHv2 - using x86 EFI boot entry

["Luis R. Rodriguez" <mcgrof@kernel.org>]

On Fri, Apr 15, 2016 at 05:03:07PM +0100, George Dunlap wrote:
> On 15/04/16 16:30, Luis R. Rodriguez wrote:
> > On Fri, Apr 15, 2016 at 10:59:16AM +0100, George Dunlap wrote:
> >> On 14/04/16 20:44, Luis R. Rodriguez wrote:
> >>> No, I meant to ask, would it be possible to make booting HVMLite using EFI
> >>> be optional ? That way if you already support EFI that can be used on
> >>> your entires with some small modifications.
> >>
> >> I wasn't talking about actual non-Linux unikernels; I was talking about using
> >> Linux in the way that unikernels are used ("unikernel-style").  That is, you
> >> boot a minimal Linux image with a small ramdisk and have a single process
> >> running as init.  For this use case, even an extra megabyte of guest RAM and
> >> an extra second of boot time is a significant cost.  "Use OVMF for domUs" is
> >> an excellent solution for traditional VMs where you boot a full distro, but
> >> would impose a significant cost on using Linux in unikernel-style VMs.
> > 
> > Understood.
> > 
> >> Whether a stripped-down EFI support would be sufficiently low memory /
> >> latency for such workloads is an open question that would take time and
> >> engineering effort to discover.  And in any case, it would certainly
> >> require the maintenance of Yet Another Bootloader in the Xen source tree.
> > 
> > OVMF is used by ARM, so using it should be a matter of adaptation, and
> > some changes other than perhaps DT use. Question still stands though,
> > would it be possible to have HVMLite be using EFI as an option so that
> > some users could opt-in if they so wish ?
> 
> Well we definitely intend go have a mode of PVH* which boots OVMF to
> EFI-enabled guests, if that's what you mean.  For one thing, that should
> in theory allow us to boot Windows guests without needing to spin up
> qemu to emulate any devices (since OVMF will be able to access the PV
> devices until the Windows PV drivers come up).

OK so for Windows x86 HVMLite will need to go the EFI boot route for sure,
only it will use OVMF ?

> Booting to EFI-enabled
> distros is certainly something we want as well.
> 
> But we need an option for dom0, and ideally we'd like an option for
> lightweight Linux guests.  It's using EFI for those purposes that we're
> pushing back on.
> 
>  -George
> 
> * I'm saying PVH because I hope when everything is sorted out we can
> just call HVMLite PVH again.

OK sure, so so long as:

 * Other OSes don't have to use EFI
 * We keep a Linux non-EFI lightweight boot mechanism

Then the OVMF / EFI route (perhaps alternatives might be minimal EFI
emulation) is still a prospect on the table long term.

  Luis