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

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Message-ID: <4AB900CC.7090409@goop.org>
Date:	Tue, 22 Sep 2009 09:52:28 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Avi Kivity <avi@...hat.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Alok Kataria <akataria@...are.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Chris Wright <chrisw@...s-sol.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"virtualization@...ts.osdl.org" <virtualization@...ts.osdl.org>,
	Greg KH <greg@...ah.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Paravirtualization on VMware's Platform [VMI].

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
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