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

[George Dunlap <george.dunlap@citrix.com>]

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).  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.