[CentOS-virt] XEN and RH 6

Pasi Kärkkäinen pasik at iki.fi
Tue Nov 10 17:33:34 UTC 2009

On Tue, Nov 10, 2009 at 07:49:01AM -0800, Grant McWilliams wrote:
>      Which I guess makes describing a guest as "fully virtualized" or
>      "paravirtualized" rather pointless given that there now is just a degree
>      of
>      how paravirtualized a guest is depending on the drivers you use.
>      Regards,
>      Â  Dennis
>    I disagree completely. KVM or Xen HVM are fully virtualized except for two
>    drivers. This is not
>    the same thing as paravirtualized. People seem to think the only thing a
>    computer does is access the
>    disk and network device. With a PV everything is running native and the
>    only overhead is from the Hypervisor.
>    In a most cases using the VT bits in the CPU makes the virtualization
>    slower in all aspects. This may not be the case
>    in the future. The developers of VirtualBox have documented this.

Yeah.. Xen paravirtualized mmu is fast, and in some (many) cases beats
CPU hardware virtualized mmu.

KVM has 'pvmmu' aswell, but it's not as good, so KVM is faster with CPU
hardware virtualization. But that's a problem of KVM only, they haven't
managed to optimize the pvmmu. And they're going to drop it altogether.

KVM people tend to say 'paravirtualized mmu is slow', but they just mean
KVM implementation of it sucks :)

-- Pasi

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