Date: Fri, 15 Apr 2016 19:17:15 +0200
From: "Luis R. Rodriguez" <mcgrof@kernel.org>
To: George Dunlap <george.dunlap@citrix.com>
Cc: "Luis R. Rodriguez" <mcgrof@kernel.org>,
        Matt Fleming <matt@codeblueprint.co.uk>, jeffm@suse.com,
        Michael Chang <MChang@suse.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jim Fehlig <jfehlig@suse.com>, Jan Beulich <JBeulich@suse.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Daniel Kiper <daniel.kiper@oracle.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        Takashi Iwai <tiwai@suse.de>,
        =?utf-8?Q?Vojt=C4=9Bch_Pavl=C3=ADk?= <vojtech@suse.cz>,
        Gary Lin <GLin@suse.com>, xen-devel <xen-devel@lists.xenproject.org>,
        Jeffrey Cheung <JCheung@suse.com>, Juergen Gross <jgross@suse.com>,
        Stefano Stabellini <stefano.stabellini@eu.citrix.com>,
        joeyli <jlee@suse.com>, Borislav Petkov <bp@alien8.de>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Charles Arndol <carnold@suse.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Julien Grall <julien.grall@arm.com>,
        Andy Lutomirski <luto@amacapital.net>,
        David Vrabel <david.vrabel@citrix.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Roger Pau =?iso-8859-1?Q?Monn=E9?= <roger.pau@citrix.com>
Subject: Re: [Xen-devel] HVMLite / PVHv2 - using x86 EFI boot entry
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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