Re: Paravirtualization on VMware's Platform [VMI].

Ingo Molnar
2009-09-22T18:03:54Z

* Jeremy Fitzhardinge <jeremy@goop.org> wrote:

> On 09/22/09 01:09, Ingo Molnar wrote:
> >>> kvm will be removing the pvmmu support soon; and Xen is talking about
> >>> running paravirtualized guests in a vmx/svm container where they don't
> >>> need most of the hooks.
> >>>
> >> We have no plans to drop support for non-vmx/svm capable processors,
> >> let alone require ept/npt.
> >
> > But, just to map out our plans for the future, do you concur with
> > the statements and numbers offered here by the VMware and KVM folks
> > that on sufficiently recent hardware, hardware-assisted
> > virtualization outperforms paravirt_ops in many (most?) workloads?
>
> Well, what Avi is referring to here is some discussions about a hybrid
> paravirtualized mode, in which Xen runs a normal Xen PV guest within a
> hardware container in order to get some immediate optimisations, and
> allow further optimisations like using hardware assisted paging
> extensions.
>
> For KVM and VMI, which always use a shadow pagetable scheme, hardware
> paging is now unambigiously better than shadow pagetables, but for Xen
> PV guests the picture is mixed since they don't use shadow pagetables.
> The NPT/EPT extensions make updating the pagetable more efficent, but
> actual access is more expensive because of the higher load on the TLB
> and the increased expense of a TLB miss, so the actual performance
> effects are very workload dependent.

obviously they are workload dependent - that's why numbers were posted
in this thread with various workloads. Do you concur with those
conclusions that they are generally a speedup over paravirt? If not,
which are the workloads where paravirt offers significant speedup over
hardware acceleration?

Ingo