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Re: [Xen-users] About virtualization, xen and kvm.

To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] About virtualization, xen and kvm.
From: "Ricardo J. Barberis" <ricardo.barberis@xxxxxxxxxxxx>
Date: Tue, 24 Feb 2009 23:51:13 -0200
Cc: Patrick Archibal <bugpb60@xxxxxxxxxxx>
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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 

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!

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

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