Re: [Xen-users] para virtualised and full virtualised

[Grok Mogger <linuximp@xxxxxxxxx>]

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*]
> >
> >
> > --
> > No virus found in this outgoing message.
> > Checked by AVG Free Edition.
> > Version: 7.5.430 / Virus Database: 268.15.3/562 - Release 
> > Date: 12/1/2006 1:12 PM
Cool, thanks for clearing all that up.

- GM


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