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

Jeremy Fitzhardinge
2009-09-22T16:52:30Z

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.

> I.e. paravirt_ops becomes a legacy hardware thing, not a core component
> of the design of arch/x86/.
>
> (with a long obsoletion period, of course.)
>

I expect we'll eventually get to the point that the performance delta
and the installed userbase will no longer justify the effort in
maintaining the full set of pvops hooks. But I don't have a good
feeling for when that might be.

J