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[Xen-devel] Re: A proposal - binary

To: David Lang <dlang@xxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] Re: A proposal - binary
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Fri, 04 Aug 2006 14:26:20 -0700
Cc: Andrew Morton <akpm@xxxxxxxx>, zach@xxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxx, jeremy@xxxxxxxxxxxxx, simon@xxxxxxxxxxxxx, jlo@xxxxxxxxxx, greg@xxxxxxxxx, Rusty Russell <rusty@xxxxxxxxxxxxxxx>, Antonio Vargas <windenntw@xxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, hch@xxxxxxxxxxxxx, ian.pratt@xxxxxxxxxxxxx, torvalds@xxxxxxxx, Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
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David Lang wrote:
how can I compile in support for Xen4 on my 2.6.18 kernel? after all xen 2 and xen3 are incompatable hypervisors so why wouldn't xen4 (and I realize there is no xen4 yet, but there is likly to be one during the time virtual servers created with 2.6.18 are still running)

Firstly, backwards compatibility is very important; I would guess that if there were a Xen4 ABI, the hypervisor would still support Xen3 for some time. Secondly, if someone goes to the effort of backporting a Xen4 paravirtops driver for 2.6.18, then you could compile it in.

I also am missing something here. how can a system be compiled to do several different things for the same privilaged opcode (including running that opcode) without turning that area of code into a performance pig as it checks for each possible hypervisor being present?

Conceptually, the paravirtops structure is a structure of pointers to functions which get filled in at runtime to support whatever hypervisor we're running over. But it also has the means to patch inline versions of the appropriate code sequences for performance-critical operations.


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