Re: HVMLite / PVHv2 - using x86 EFI boot entry

From: "Luis R. Rodriguez" <mcgrof@kernel.org>
To: David Vrabel <david.vrabel@citrix.com>,
	Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: "Matt Fleming" <matt@codeblueprint.co.uk>,
	"Michael Chang" <MChang@suse.com>,
	linux-kernel@vger.kernel.org,
	"Julien Grall" <julien.grall@arm.com>,
	"Jan Beulich" <JBeulich@suse.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Daniel Kiper" <daniel.kiper@oracle.com>,
	x86@kernel.org, "Vojtěch Pavlík" <vojtech@suse.cz>,
	"Gary Lin" <GLin@suse.com>,
	xen-devel@lists.xenproject.org,
	"Jeffrey Cheung" <JCheung@suse.com>,
	"Charles Arndol" <carnold@suse.com>, joeyli <jlee@suse.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Boris Ostrovsky" <boris.ostrovsky@oracle.com>,
	"Juergen Gross" <jgross@suse.com>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Jim Fehlig" <jfehlig@suse.com>,
	"Andy Lutomirski" <luto@amacapital.net>,
	"Luis R. Rodriguez" <mcgrof@kernel.org>,
	"Linus Torvalds" <torvalds@linux-foundation.org>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: HVMLite / PVHv2 - using x86 EFI boot entry
Date: Fri, 8 Apr 2016 22:40:32 +0200	[thread overview]
Message-ID: <20160408204032.GR1990__25804.0717257702$1460148102$gmane$org@wotan.suse.de> (raw)
In-Reply-To: <5704D978.1050101@citrix.com>

On Wed, Apr 06, 2016 at 10:40:08AM +0100, David Vrabel wrote:
> On 06/04/16 03:40, Luis R. Rodriguez wrote:
> > 
> >     * You don't need full EFI emulation
> 
> I think needing any EFI emulation inside Xen (which is where it would
> need to be for dom0) is not suitable because of the increase in
> hypervisor ABI.

Is this because of timing on architecture / design of HVMLite, or
a general position that the complexity to deal with EFI emulation
is too much for Xen's taste ?

ARM already went the EFI entry way for domU -- it went the OVMF route,
would such a possibility be possible for x86 domU HVMLite ? If not why
not, I mean it would seem to make sense to at least mimic the same type
of early boot environment, and perhaps there are some lessons to be
learned from that effort too.

Are there some lessons to be learned with ARM's effort? What are they?
If that could be re-done again with any type of cleaner path, what
could that be that could help the x86 side ?

Although emulating EFI may require work, some folks have pointed out
that the amount of work may not be that much. If that is done can
we instead rely on the same code to replace OVMF to support both
Xen ARM and Xen HVMLite on x86 ? What would be the pros / cons of
this ?

> I also still do not understand your objection to the current tiny stub.

Its more of a hypothetical -- can an EFI entry be used instead given
it already does exactly what the new small entry does ? Its also rather
odd to add a new entry without evaluating fully a possible alternative
that would provide the same exact mechanism.

A full technical unbiased evaluation of the different approaches is what I'd
hope we could strive to achieve through discussion and peer review, thinking
and prioritizing ultimately what is best to minimize the impact on Linux
and also help take advantage of the best features possible through both
means. Thinking long term, not immediate short term.

  Luis

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