linux-kernel - Re: HVMLite / PVHv2 - using x86 EFI boot entry

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Date:	Wed, 13 Apr 2016 11:40:03 +0100
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	Roger Pau Monné <roger.pau@...rix.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Juergen Gross <jgross@...e.com>,
	"Luis R. Rodriguez" <mcgrof@...nel.org>,
	David Vrabel <david.vrabel@...rix.com>,
	Julien Grall <julien.grall@....com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Charles Arndol <carnold@...e.com>,
	Jim Fehlig <jfehlig@...e.com>, Jan Beulich <JBeulich@...e.com>,
	Daniel Kiper <daniel.kiper@...cle.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	Gary Lin <GLin@...e.com>, Borislav Petkov <bp@...en8.de>,
	joeyli <jlee@...e.com>, Jeffrey Cheung <JCheung@...e.com>,
	Michael Chang <MChang@...e.com>,
	Vojtěch Pavlík <vojtech@...e.cz>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: HVMLite / PVHv2 - using x86 EFI boot entry

On Wed, 13 Apr, at 11:15:15AM, Matt Fleming wrote:
> 
> For 1. we'd basically be using the PE/COFF file format with the EFI
> ABI as an OS agnostic boot protocol, but not as a full firmware
> runtime environment.

To add some balance to this proposal (since there's no such thing as a
free lunch) some of the disadvantages are,

The PE/COFF stub in Linux does assume that it is executing in native
cpu mode and does not perform any mode switching, i.e. from 32-bit
protected to long mode. This is due to the way that EFI works - by the
time the OS image entry point is jumped to on a 64-bit cpu we're
running in long mode with identity mapped page tables. To be fair,
when running Xen on EFI (bare metal) this would save you one cpu mode
switch when compared with the current HVMLite proposal.

I'm not aware of a direct equivalent for ELF notes in the PE/COFF
format. I'm still re-reading the spec to find something suitable.