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xen-users
Re: [Xen-users] para virtualised and full virtualised
Petersson, Mats wrote:
-----Original Message-----
From: Grok Mogger [mailto:linuximp@xxxxxxxxx]
Sent: 01 December 2006 16:52
To: Petersson, Mats
Cc: anant; xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] para virtualised and full virtualised
Petersson, Mats wrote:
[*snip*]
2) My understanding about running Xen "fully virtualized" is
that it requires 3 things.
i) Your processor must support Virtualization Technology
(VT) (called "Intel VT" by Intel or "AMDV" by AMD).
ii) Your BIOS must support VT. If it does, you'll have some
options to turn VT on and off from within the BIOS.
Currently, AMD doesn't support "switching off" AMD-V, so it's not
dependant on any BIOS features - it's only dependant on having "the
right model of processor".
Thanks for correcting me about the BIOS. So just to be clear,
the BIOS has absolutely nothing to do with it, right? The
processor is all that matters.
For AMD processors, the BIOS can not (for the currently available
processors) disable the AMD-V feature - which means that it doesn't
matter what BIOS you use on the machine. Future versions of the
processor may not follow this pattern.
I know that Intel claims on their website that your BIOS must
support VT, but I've seen so many people suggest otherwise now
that I'm assuming that must be false.
Intel processors come up with VMX disabled, so it's not going to work if
the BIOS doesn't do the right things to ENABLE it.
iii) Xen must be compiled to support VT. I think I recall
seeing some instructions about how to compile Xen to support
full virtualization somewhere in the Xen 3.0 manual...
There are a few things that need to be done to build for
HVM (VT) mode,
but the actual Xen kernel supports HVM by and of itself without any
settings or adjustments - just build the code and it's there.
The things that need special attention is the fact that to boot a
fully-virtualized domain (guest), you'll need a BIOS that
loads the OS
itself (just like you need for a REAL machine). The BIOS requires a
special compiler to be downloaded. I believe this is now an
ERROR rather
than a WARNING, but I'm not 100% sure. Either way, if
you're not certain
if this works, go to <xen-source-dir>/tools/firmware and do
"make". If
it builds ok and you have a hvmloader executable in the hvmloader
direectory, then you've got all the tools. If that's not
the case, then
you'll need to follow the instructons from the makefile.
So you're NOT referring to the BIOS that is actually on your
motherboard in a flash rom here, right? You mean that you need
to have a separate BIOS resident on the hard disk (which will
appear as an executable called "hvmloader"). Could you point me
to where this is documented? I don't recall seeing it anywhere.
Correct, it's not the BIOS that sits on the motherboard.
This BIOS is built as part of the hvmloader build process, and is then
included in the "roms.h" that is compiled into hvmloader itself.
"roms.h" contains the binary images for the BIOS "rom" that is then
loaded into the memory of the virtual machine (within the guest's
address space, they are loaded at the address where these images usually
lives in the MACHINE physical space, so the BIOS is loaded at
0xF0000-0xFFFFF for example).
So you won't see a BIOS image in your directory structure unless you
look in the source-directories.
Not sure if it's "documented" in any great way anywhere.
Also, what kind of special compiler would I need? Where could I
download it? This is the first time I've heard any mention of that.
If you go to <xen-source-dir>/tools/firmware and type "make", if you
haven't got "bcc", it will tell you to download it from
http://www.cix.co.uk/~mayday .
You need this compiler because normal GCC is not capable of building
16-bit code. That's what bcc is for.
(In case you haven't figured out, I've never actually used full
virtualization; I don't have hardware for it at the moment.
I'm running Xen with paravirtualization, but I'd like to go
ahead and figure this HVM stuff out for the future. =) )
3) I believe you can determine whether or not your processor
supports Intel VT by going to looking at the text file
"/proc/cpuinfo". In that file, you will see a line that starts
with the word "flags". The line will most likely also contain a
list of 3 or 4 letter abbreviations that (to me at least) mostly
don't make a whole lot of sense. If you see "vmx" in that list,
supposedly that means that you have a VT enabled processor. I
have read this a few places on the internet, but am a little
skeptical myself.
For AMD processors, you'll need to look for "svm", "vmx" is
the Intel
name for this feature.
Okay, cool. =)
Oh! One more thing though. At the moment, Xen does not support
AMD processors with VT, right? It only works with Intel VT
enabled processors. Is that right?
Not right. AMD processors have been supported since version 3.0.1 of Xen
(earlier if you used unstable, of course).
--
Mats
Thanks for replying,
-GM
[*snip*]
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Cool, thanks for clearing all that up.
- GM
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PM
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