Xen users - Jan 2007 - RE: Config: Paravirtualization and Full Virtualization

Where''s the difference in the guest installation (or maybe in the host
installation?) when you install a Linux in paravirtualization mode, or
in full virtualization mode (VT or pacifica)?

g>> Full gives better performance from what I hear.  Your hardware
should be suitable and the bios option enabled.
If you know anything about bios pls do let me know as Sony have disabled this
option in there bios.

I mean, how do you tell to xen that you want to run a linux guest in
paravirtualization or in full virtualization mode?

g>> there are different config scripts under /etc/xen/scripts.  #
man xm will get you started.

And then, can you run a linux (modified to run under
paravirtualization) in full virtualization mode?

g>> check your bios and config scripts

Can I run a linux in paravirtualization mode and another in full
virtualization mode, both simultanously?

g>> well strange as this may sound i have not successfully
configured a xen machine yet.  I''m getting a destroyDevice error...
anyway from what I understand the answer to this question is yes and again
depending on your bios.

Hope this helps
Graeme

 ----- Original Message ------
 From:Jordi Segues
 Sent:Tuesday, January 30, 2007 17:48
 To: xen-users@lists.xensource.com; 
 Subject:[Xen-users] Config: Paravirtualization and Full Virtualization

Hello,

I''m playing with xen, but I have a doubt:

Where''s the difference in the guest installation (or maybe in the host
installation?) when you install a Linux in paravirtualization mode, or
in full virtualization mode (VT or pacifica)?

I mean, how do you tell to xen that you want to run a linux guest in
paravirtualization or in full virtualization mode?

And then, can you run a linux (modified to run under
paravirtualization) in full virtualization mode?

Can I run a linux in paravirtualization mode and another in full
virtualization mode, both simultanously?

Thanks a lot!
And sorry if this is a basic question.

-- 
Jordi Segués Daina

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On Wed, 2007-01-31 at 11:54 +0200, Graeme Gerber wrote:
> Where''s the difference in the guest installation (or maybe in the
host
> installation?) when you install a Linux in paravirtualization mode, or
> in full virtualization mode (VT or pacifica)?
> 
> g>> Full gives better performance from what I hear.  Your hardware
> should be suitable and the bios option enabled.
> If you know anything about bios pls do let me know as Sony have
> disabled this option in there bios.
Full gives better performance for the Xen client - but since Para does
not require complete hardware emulation (and requires optimized Xen
kernels), it results in better performance overall, especially if you
have multiple Xen clients.
> 
Many laptop mfrs disable VT, even if the Intel CPU supports it (AMD
VT-capable systems can''t have VT disabled). There''s a long HP
thread on
the topic (which is also covered in /.)
http://forums.itrc.hp.com/service/forums/bizsupport/questionanswer.do?threadId=1051601

There are also interesting Wikis on the topic
http://wiki.xensource.com/xenwiki/IntelVT
http://wiki.xensource.com/xenwiki/HVM_Compatible_Notebooks

Thanks,
Mike


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> -----Original Message-----
> From: xen-users-bounces@lists.xensource.com 
> [mailto:xen-users-bounces@lists.xensource.com] On Behalf Of MJang
> Sent: 01 February 2007 14:52
> To: xen-users@lists.xensource.com
> Subject: RE: [Xen-users] Config: Paravirtualization and Full 
> Virtualization
> 
> On Wed, 2007-01-31 at 11:54 +0200, Graeme Gerber wrote:
> > Where''s the difference in the guest installation (or maybe 
> in the host
> > installation?) when you install a Linux in 
> paravirtualization mode, or
> > in full virtualization mode (VT or pacifica)?
> > 
> > g>> Full gives better performance from what I hear.  Your
hardware
> > should be suitable and the bios option enabled.
> > If you know anything about bios pls do let me know as Sony have
> > disabled this option in there bios.
> 
> Full gives better performance for the Xen client - but since Para does
> not require complete hardware emulation (and requires optimized Xen
> kernels), it results in better performance overall, especially if you
> have multiple Xen clients.
Even if you don''t touch any IO hardware, I don''t think
hardware
virtualization is noticably faster (if faster at all) than
Para-virtualization (and I have a good reason to NOT say this, but I say
it, because I believe it''s the case, currently at least). Of course if
the guest is doing absolutely nothing that the hypervisor needs to know
about, there''s very little difference in the two cases, as
it''s 99.9%
about the actual speed of the system itself (CPU and memory, as other
components, such as disk and network, are controlled via the hypervisor
in one way or another). But assuming we''re running something that
doesn''t do disk-access or network-access, but needs a bit of help from
the hypervisor for other aspects, such as memory management, I would say
that para-virtual is either going to be faster or same speed for the
same task. 

One thing that will change this is the ability to use "Nested paging"
-
that will allow the hypervisor to give the VM it''s own memory region,
mapping for example 0..256MB of "guest memory" to a section of
"machine
memory" that is 256MB somewhere in the machine. By this extension to the
architecture (which is already in the AMD specs), it''s possible for the
guest to run almost autonomously with a very small overhead. A hybrid of
this technique and para-virtualization is also technically possible,
where a very thin/small hardware virtualization layer is used in
conjunction with an otherwise para-virtual OS - that way achieving the
best of both worlds. 

There are other reasons to use full virtualization today, and one of
those is the inability to xenify all available operating systems, either
due to lack of available source code or lack of resources.

--
mats



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On Thu, 2007-02-01 at 16:14 +0100, Petersson, Mats
wrote:
>  
> > -----Original Message-----
> > From: xen-users-bounces@lists.xensource.com 
> > [mailto:xen-users-bounces@lists.xensource.com] On Behalf Of MJang
> > Sent: 01 February 2007 14:52
> > To: xen-users@lists.xensource.com
> > Subject: RE: [Xen-users] Config: Paravirtualization and Full 
> > Virtualization
> > 
> > On Wed, 2007-01-31 at 11:54 +0200, Graeme Gerber wrote:
> > > Where''s the difference in the guest installation (or
maybe
> > in the host
> > > installation?) when you install a Linux in 
> > paravirtualization mode, or
> > > in full virtualization mode (VT or pacifica)?
> > > 
> > > g>> Full gives better performance from what I hear.  Your
hardware
> > > should be suitable and the bios option enabled.
> > > If you know anything about bios pls do let me know as Sony have
> > > disabled this option in there bios.
> > 
> > Full gives better performance for the Xen client - but since Para does
> > not require complete hardware emulation (and requires optimized Xen
> > kernels), it results in better performance overall, especially if you
> > have multiple Xen clients.
> 
> Even if you don''t touch any IO hardware, I don''t think
hardware
> virtualization is noticably faster (if faster at all) than
> Para-virtualization (and I have a good reason to NOT say this, but I say
> it, because I believe it''s the case, currently at least). Of
course if
> the guest is doing absolutely nothing that the hypervisor needs to know
> about, there''s very little difference in the two cases, as
it''s 99.9%
> about the actual speed of the system itself (CPU and memory, as other
> components, such as disk and network, are controlled via the hypervisor
> in one way or another). But assuming we''re running something that
> doesn''t do disk-access or network-access, but needs a bit of help
from
> the hypervisor for other aspects, such as memory management, I would say
> that para-virtual is either going to be faster or same speed for the
> same task. 
> 
> One thing that will change this is the ability to use "Nested
paging" -
> that will allow the hypervisor to give the VM it''s own memory
region,
> mapping for example 0..256MB of "guest memory" to a section of
"machine
> memory" that is 256MB somewhere in the machine. By this extension to
the
> architecture (which is already in the AMD specs), it''s possible
for the
> guest to run almost autonomously with a very small overhead. A hybrid of
> this technique and para-virtualization is also technically possible,
> where a very thin/small hardware virtualization layer is used in
> conjunction with an otherwise para-virtual OS - that way achieving the
> best of both worlds. 
> 
> There are other reasons to use full virtualization today, and one of
> those is the inability to xenify all available operating systems, either
> due to lack of available source code or lack of resources.
Dear Mats, 

I appreciate the clarifications. I get the vague impression that there
are serious debates on the issue w/r/t who or what config saves more
resources. I gave my impressions, which could very well be wrong. But I
bow to your greater expertise.

Thanks,
Mike


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