From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeremy Fitzhardinge Subject: Re: Paravirtualization on VMware's Platform [VMI]. Date: Tue, 22 Sep 2009 09:52:28 -0700 Message-ID: <4AB900CC.7090409@goop.org> References: <1253233028.19731.63.camel@ank32.eng.vmware.com> <20090919224430.GB9567@kroah.com> <1253419185.3253.21.camel@ank32.eng.vmware.com> <20090920074247.GA5733@elte.hu> <20090920095239.456ad6f2@infradead.org> <4AB5EF25.9070502@redhat.com> <4AB64EFC.10707@goop.org> <20090922080913.GB1475@elte.hu> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20090922080913.GB1475@elte.hu> Sender: linux-kernel-owner@vger.kernel.org To: Ingo Molnar Cc: Avi Kivity , Arjan van de Ven , Alok Kataria , Thomas Gleixner , "H. Peter Anvin" , the arch/x86 maintainers , LKML , Chris Wright , Rusty Russell , "virtualization@lists.osdl.org" , Greg KH , Linus Torvalds , Andrew Morton List-Id: virtualization@lists.linuxfoundation.org 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