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

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Date:	Tue, 22 Sep 2009 20:02:16 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
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].


* Jeremy Fitzhardinge <jeremy@...p.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
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