linux-kernel - Re: [Xen-devel] HVMLite / PVHv2

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Message-ID: <20160414194408.GP1990@wotan.suse.de>
Date:	Thu, 14 Apr 2016 21:44:08 +0200
From:	"Luis R. Rodriguez" <mcgrof@...nel.org>
To:	George Dunlap <george.dunlap@...rix.com>
Cc:	"Luis R. Rodriguez" <mcgrof@...nel.org>,
	Matt Fleming <matt@...eblueprint.co.uk>, jeffm@...e.com,
	Michael Chang <MChang@...e.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jim Fehlig <jfehlig@...e.com>, Jan Beulich <JBeulich@...e.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Daniel Kiper <daniel.kiper@...cle.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Takashi Iwai <tiwai@...e.de>,
	Vojtěch Pavlík <vojtech@...e.cz>,
	Gary Lin <GLin@...e.com>,
	xen-devel <xen-devel@...ts.xenproject.org>,
	Jeffrey Cheung <JCheung@...e.com>,
	Juergen Gross <jgross@...e.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	joeyli <jlee@...e.com>, Borislav Petkov <bp@...en8.de>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Charles Arndol <carnold@...e.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Julien Grall <julien.grall@....com>,
	Andy Lutomirski <luto@...capital.net>,
	David Vrabel <david.vrabel@...rix.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Roger Pau Monné <roger.pau@...rix.com>
Subject: Re: [Xen-devel] HVMLite / PVHv2 - using x86 EFI boot entry

On Thu, Apr 14, 2016 at 10:53:47AM +0100, George Dunlap wrote:
> On 13/04/16 20:52, Luis R. Rodriguez wrote:
> > On Wed, Apr 13, 2016 at 04:44:54PM +0100, George Dunlap wrote:
> >> On Thu, Apr 7, 2016 at 7:51 PM, Luis R. Rodriguez <mcgrof@...e.com> wrote:
> >>> So more to it, if the EFI entry already provides a way into Linux
> >>> in a more streamlined fashion bringing it closer to the bare metal
> >>> boot entry, why *would* we add another boot entry to x86, even if
> >>> its small and self contained ?
> >>
> >> We would avoid using EFI if:
> > 
> > And this is what I was looking for, thanks!
> > 
> >> * Being called both on real hardware and under Xen would make the EFI
> >> entry point more complicated
> > 
> > That's on the EFI Linux maintainer to assess. And he seems willing to
> > consider this.
> > 
> >> * Adding the necessary EFI support into Xen would be a significant
> >> chunk of extra work
> > 
> > This seems to be a good sticking point, but Andi noted another aspect
> > of this or redundancy as well.
> > 
> >> * Requiring PVH mode to implement EFI would make it more difficult for
> >> other kernes (NetBSD, FreeBSD) to act as dom0s.
> > 
> > What if this is an option only then ?
> > 
> >>
> >> * Requiring PVH mode to use EFI would make it more difficult to
> >> support unikernel-style workloads for domUs.
> > 
> > What if this is an option only then ?
> 
> So first of all, you asked why anyone would oppose EFI, and this is part
> of the answer to that.
> 
> Secondly, you mean "What if this is the only thing the Linux maintainers
> will accept?"  And you already know the answer to that.

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.

> How much of a burden it would be on the rest of the open-source
> ecosystem (Xen, *BSDs, &c) is a combination of some as-yet unknown facts
> (i.e., what a minimal Xen/Linux EFI interface would look like) and a
> matter of judgement (i.e., given the same interface, reasonable people
> may come to different conclusions about whether the interface is an
> undue burden to impose on others or not).
> 
> But I would hope that the Linux maintainers would at least consider the
> broader community when weighing their decisions, and not take advantage
> of their position of dominance to simply ignore the effect of their
> choices on everybody else.

This has nothing to do with dominance or anything nefarious, I'm asking
simply for a full engineering evaluation of all possibilities, with
the long term in mind. Not for now, but for hardware assumptions which
are sensible 5 years from now.

  Luis