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RE: [Xen-devel] x86-64 32bit applications support discussion

To: "Jerone Young" <jyoung5@xxxxxxxxxx>, "xen-devel" <xen-devel@xxxxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel] x86-64 32bit applications support discussion
From: "Nakajima, Jun" <jun.nakajima@xxxxxxxxx>
Date: Mon, 14 Mar 2005 13:59:34 -0800
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Thread-topic: [Xen-devel] x86-64 32bit applications support discussion
Jerone Young wrote:
>       This thread is intended for discussion of 32bit application
> under x86-64 virtual domains. To all reading this is for development
> purposes (x86-64 support is not done yet).
>       I've been actively looking into 32bit application support for
> x86-64 Virutal Domains. I do have a question that I wanted to bring to
> the list to start the discussion.
>       Since the virtual domain & applications are all running in ring
> Does the hypervisor need to know how to tell the CPU when and when
> not to go into compatibility mode to  run running 32bit applications
> properly under virtual domains? As you may know while under Long mode
> you have two sub modes: 64-bit mode & compatibility (for 32bit
> stuff). The problem I see is that since we are in Long Mode the
> virutal domain needs to tell CPU to go into the sub mode
> compatibility mode for it's 32 applications but does not have the
> access to do so. I'm not sure the best way of going about this (new
> hypercall?) ? 
The switch_to_user hypercall should allow you to switch to compatibility
mode as long as you provide one for that. Look at struct switch_to_user
and the do_switch_to_user. Xenolinux will set the same cs as the native
x86-64 Linux does for 32-bit processes. 


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