linux-kernel - Re: Merge Xen (the hypervisor) into Linux

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Date:	Wed, 03 Jun 2009 13:41:58 +0200
From:	Gerd Hoffmann <kraxel@...hat.com>
To:	Theodore Tso <tytso@....edu>, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	George Dunlap <george.dunlap@...citrix.com>,
	David Miller <davem@...emloft.net>, jeremy@...p.org,
	avi@...hat.com, xen-devel@...ts.xensource.com, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Keir Fraser <Keir.Fraser@...citrix.com>,
	torvalds@...ux-foundation.org, gregkh@...e.de,
	kurt.hackel@...cle.com, Ian Pratt <Ian.Pratt@...citrix.com>,
	xen-users@...ts.xensource.com,
	ksrinivasan <ksrinivasan@...ell.com>, EAnderson@...ell.com,
	wimcoekaerts@...mekes.net,
	Stephen Spector <stephen.spector@...rix.com>,
	jens.axboe@...cle.com, npiggin@...e.de
Subject: Re: Merge Xen (the hypervisor) into Linux

On 06/03/09 13:15, Theodore Tso wrote:
> On Wed, Jun 03, 2009 at 11:09:39AM +0200, Gerd Hoffmann wrote:
>> It wasn't my intention to imply that.  The interface can be extended
>> when needed.  PAT support will probably be such a case.  Changing it in
>> incompatible ways isn't going to work though.
>
> But that means that if there is some fundamentally broken piece of
> dom0 design, that the Linux kernel will be stuck with it ***forever***
> and it will contaminate code paths and make the code harder to
> maintain ***forever*** if we consent to the Xen merge?

No.  Xen is stuck with it forever (or at least for a few releases). 
Even when adding new & better dom0/xen interfaces in the merge process 
Xen has to keep the old ones to handle the other dom0 guests (NetBSD, 
Solaris, old 2.6.18 out-of-tree linux kernel).  Pretty much like the 
linux kernel has to keep old syscalls to not break the ABI for the 
applications, xen has to maintain old hypercalls[1].

Other way around:  Apps can use new system calls only when running one 
recent kernels, and they have to deal with -ENOSYS.  Likewise it might 
be that the pv_ops-based dom0 kernel can provide some features only when 
running on a recent hypervisor.  That will likely be the case for PAT.

cheers,
   Gerd

[1] and other interfaces like trap'n'emulate certain instructions.
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