Xen users - Feb 2009 - About virtualization, xen and kvm.

Hello,

I have understand that :

----------------------------
           guest
----------------------------
    driver_device0_guest
----------------------------
         hypervisor
----------------------------
  driver_device0_hypervisor
----------------------------
         device0
----------------------------

in order to access to a device, a guest use driver_device0_guest
The driver_device0_guest is modified to ask the hypervisor instead of the
device0 directly.
Then the hypervisor uses the driver_device0_hypervisor in order to ask the
device0.

It''s my understant, maybe it''s false.

I have few questions :

1 - what is the difference between libvirt and virtio.

2 - why developpers for xen, kvm... must develop drivers each time there are new
devices ?
Because the driver must ask the hypervisor instead of device directly.

3 - The devices presented to the guest are real or emulated ? (for xen and/or
kvm)
if devices are real, for example, i can use my graphic card with a virtual
machine ??

4 - Can you tell me why kvm is best or xen is best in enterprise.

In my understand, in the future, kvm is in good position because the
developpment is integrated in the kernel. Also, i feel xen progress because
firms like citrix contribute but it''s seems there is no big update
since few years.

My vision is xen is best for production environnement because it takes
advantages of minimal hypervisor code while kvm must be install with the kernel.

Also, it seems, performance will be better in kvm with virtio. Can we use virtio
with xen ?
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Patrick Archibal wrote:
> 4 - Can you tell me why kvm is best or xen is best in enterprise.
> 
> In my understand, in the future, kvm is in good position because the
> developpment is integrated in the kernel.
This is what people working on KVM are trying to let us believe, but
this is not truth. There IS a LOT of work, so that Xen dom0 support is
integrated in the kernel.org kernel. People from Xen like Ian Prat said
last summer they were aiming at kernel 2.6.28, now they are pushing so
that the future 2.6.29 integrates dom0 support (in both 32 and 64 bits).
Even if they miss the 2.6.29 deadline, the 2.6.30 might be reached (each
kernel release are made approximatively each 3 months).
> Also, i feel xen progress
> because firms like citrix contribute but it''s seems there is no
big
> update since few years.
Not really. Look a bit more in the dev. list of Xen, and you will see
that so many people are contributing. For example from HP, Fujitsu and
others.
> My vision is xen is best for production environnement because it takes
> advantages of minimal hypervisor code while kvm must be install with the
> kernel.
Sorry, but this is quite wrong as well. The very best advantage of KVM
is that it''s directly in the kernel, so it goes faster. Under Xen, when
a userland tool wants to access some hardware, it has to do userland ->
hypervisor -> dom0 driver, while in KVM the hypervisor stage is skipped.
But that is luckily not all, and Xen is in many ways better and faster,
as much as I have tested.
> Also, it seems, performance will be better in kvm with virtio. Can we
> use virtio with xen ?
That I''m not sure. Is it what I just wrote above?

Thomas


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My $0.02... and I might be horribly wrong or over simplifyng, so please 
someone enlighten us!

El Martes 24 Febrero 2009, Patrick Archibal escribió:
> Hello,
>
> I have understand that :
>
> ----------------------------
>            guest
> ----------------------------
>     driver_device0_guest
> ----------------------------
>          hypervisor
> ----------------------------
>   driver_device0_hypervisor
> ----------------------------
>          device0
> ----------------------------
>
> in order to access to a device, a guest use driver_device0_guest
> The driver_device0_guest is modified to ask the hypervisor instead of the
> device0 directly. Then the hypervisor uses the driver_device0_hypervisor in
> order to ask the device0.
>
> It''s my understant, maybe it''s false.
>
> I have few questions :
>
> 1 - what is the difference between libvirt and virtio.
Not sure what you mean with virtio, to me it''s I/O virtualization,
which can
be different things depending on the context but it generally means
"don''t
fully emulate the devices but instead provide better performant virtual 
devices/drivers".

Libvirt is an abstraction layer in the form of a library, a daemon and 
userland tools, that support several hypervisors/emulators like xen and kvm.
> 2 - why developpers for xen, kvm... must develop drivers each time there
> are new devices ? Because the driver must ask the hypervisor instead of
> device directly.
They don''t, the kernel developers do it, xen and kvm just use those
drivers.
> 3 - The devices presented to the guest are real or emulated ? (for xen
> and/or kvm) if devices are real, for example, i can use my graphic card
> with a virtual machine ??
Again, it depends. With xen HVM guests they are emulated by means of qemu but 
for example you can use James Harper''s GPL paravirtualized drivers for 
Windows guests.

To share a physical device, the device itself should support some form of 
virtualization, just like CPUs with Intel VT o AMD Pacifica do.

With xen and VT-d capable hardware you can hide a device from xen and pass it 
to a guest to use it, so you couldn''t give your guest your *only*
graphics
card, but if you have two you can lend it one of them.
> 4 - Can you tell me why kvm is best or xen is best in enterprise.
I can''t but I have read that xen is preferred over kvm in enterprise,
maybe
just because xen has better support and has been around longer than kvm.
> In my understand, in the future, kvm is in good position because the
> developpment is integrated in the kernel.
That would give it more immediate presence yes, but as Thomas pointed out, xen 
will soon be more integrated in mainline kernel.
> Also, i feel xen progress because firms like citrix contribute but
it''s
> seems there is no big update since few years.
You''re not looking hard enough :-)

Xen is very active, a few weeks ago 3.31 was relased and a few weeks before 
that 3.3.0 saw the light too.
> My vision is xen is best for production environnement because it takes
> advantages of minimal hypervisor code while kvm must be install with the
> kernel.
Xen also depends on the kernel, for drivers, etc.

And kvm is actually "more minimal" than xen in that kvm is a module 
that "converts" a running Linux kernel into a hypervisor.
> Also, it seems, performance will be better in kvm with virtio. Can we use
> virtio with xen ?
Yes, with a xen-enabled kernel in the guest or GPLPV drivers in HVM guest.

Hope it helps, and if not please fell free to correct me!

Cheers,
-- 
Ricardo J. Barberis
Senior SysAdmin - I+D
Dattatec.com :: Soluciones de Web Hosting

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