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Re: [Xen-devel] VMX Assist and x86 segment registers

To: Randy Thelen <rthelen@xxxxxxxxxx>
Subject: Re: [Xen-devel] VMX Assist and x86 segment registers
From: Anthony Liguori <aliguori@xxxxxxxxxx>
Date: Wed, 31 May 2006 10:07:08 -0500
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Big real mode is definitely a problem in vm86 mode. FreeBSD is also notorious for its use of segmentation :-) I think it has been proposed that a potential solution is to do away with vmxassist entirely and instead use qemu to emulate 16 bit mode (or another emulator if available). Not sure if anyone is actually working on that yet though. It seems to be the most promising solution.


Anthony Liguori

Randy Thelen wrote:
Executive summary --

If you're interested in running FreeBSD in an HVM domain on VT-x hardware, please EXPERIMENT with the attached patch. However, the patch is not in and of itself a fix. I'm simply looking for more help on solving this bigger problem and pushing the patch to the Xen community is the best way I could think of to get more insight into the problem.

If you're still interested, please continue reading.

Folks --

Here's an esoteric topic: x86 segment registers and their emulation with vmxassist. Anybody interested in engaging in this one? ;-)

Here's the story. FreeBSD doesn't boot on Intel processors with VT-x hardware in an HVM domain. It turns out I'm dependent upon that functionality. So, I began to investigate. Dragons lie in them thar hills. (Snakes, spiders, whatever: I -hate- segmentation and I -hate- segment registers. So you can imagine the frustration I had as I realized I needed to become intimately familiar with them to solve my problem.)

Here's the deal, the x86 processor allows segment registers to be set in one mode (e.g., "real mode") and the used in another mode (e.g., "protected mode"). For an example of how this trick is utilized, peruse subject 15: Accessing 4 Gigs of Memory in Real Mode:


(Be prepared to be completely disgusted.)

At any rate, between my own debugging and reading articles of the sort above, I realized that vmxassist was incorrectly handling segment registers.

I've made a patch that I've attached but I am -NOT- recommending this for general consumption. This patch is -ONLY- recommended for those who want to run FreeBSD on VT-x hardware and those who are willing to work through additional bugs to solve interesting problems. (If you have an interest in running other OSes on VT-x hardware, you're free to try the patch, but I can't say that I'll spend much time trying to solve the problems you bump into.)

I believe that the current handling of segment registers in vmxassist isn't quite right. A more complicated model needs to be developed. I'm not sure exactly how different it needs to be or exactly in what ways it needs to be modified.

But, if this topic interests you, I'd be interested in continuing this discussion.

Once you apply the patch, you'll need to perform a make in the xen/tools/firmware directory. That will cause a new hvmloader to be constructed. You can either install it (I don't recommend it) or you can modify your configuration file(s) so that the "kernel" is this new hvmloader file:

kernel = "/usr/src/xen-3.0-testing/tools/firmware/hvmloader/hvmloader"

If you've read this far, you might take interest in rereading my post on this problem:


-- Randy Thelen

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