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[Xen-devel] Re: Question about virtual bios in xen

To: henanwxr <henanwxr@xxxxxxxxxxx>
Subject: [Xen-devel] Re: Question about virtual bios in xen
From: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Date: Thu, 26 May 2011 20:19:11 +0100
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Hi, henanwxr, 

At 17:59 +0100 on 26 May (1306432756), henanwxr wrote:
> Hi, Tim. I have some questions.

Please post your questions on xen-devel instead of emailing people
directly.  That way, if I don't know the answer, someone else might; and
if I do reply, the answer will be in the archives for other people to

> when user create the virtual machine, then:
> 1. Which code execute firstly, hvmloaer or qemu-dm?I have traced the
> codes flow and found qemu-dm run before hvmloader,right?

Qemu-dm starts running before HVMloader, but after that they run at the
same time.  HVMloader runs inside the virtual machine, and qemu-dm runs
in dom0.

> 2. And virtual bios code are both in hvmloader and qemu-dm, but I
> think it should be only one of virtual bios will be used to initialize
> the virutal device in qemu-dm.I find at the end of hvmloader it will
> jmp to 0xffff0, that is the bios entry,so I think the virtual bios in
> hvmloader will function and virtual bios in qemu-dm don't execute.

Yes, exactly.  If there is any BIOS code in qemu-dm, ignore it.  Under
Xen, the BIOS that runs inside a virtual machine is the one in

> But how qemu-dm ingore the execution of its virual bios inside?

I don't know exactly how.  Presumably the code in qemu-dm that would
load a BIOS into guest memory isn't called in the xen case.  Perhaps you
could read the qemu code if you want to know the details. 


Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Xen Platform Team
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

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