From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: State of current pv_ops backend for KVM? Date: Wed, 30 May 2007 08:01:19 +0300 Message-ID: <465D051F.20706@qumranet.com> References: <465B87CE.6030801@codemonkey.ws> <465C12C0.9030906@qumranet.com> <465C2F75.3080603@codemonkey.ws> <465C31F2.8080007@qumranet.com> <1180492544.30202.72.camel@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm-devel To: Rusty Russell Return-path: In-Reply-To: <1180492544.30202.72.camel-bi+AKbBUZKY6gyzm1THtWbp2dZbC/Bob@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: kvm-devel-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org Errors-To: kvm-devel-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org List-Id: kvm.vger.kernel.org Rusty Russell wrote: > On Tue, 2007-05-29 at 17:00 +0300, Avi Kivity wrote: > >> Anthony Liguori wrote: >> >>> Avi Kivity wrote: >>> >>>> I think we should aim for a full direct paging implementation, where >>>> the cr3 cache is just the cherry on top that makes context switches >>>> proceed at native speed. Jun has a paper on it for OLS and I hope >>>> patches too. >>>> >>> That's a bit more ambitious than I was thinking. I would be very >>> happy with a pv timer, interrupt controller, and some basic CPU >>> optimizations. I'm still not convinced the perf benefits of direct >>> paging justify the annoyances that come with it (guests being aware of >>> pfns). >>> >> I thought that with paravirt_ops, all the awareness is limited to the >> paravirt implementation, and nothing in the core kernel is actually >> aware of host pfns. >> > > It is, but now you've broken the abstraction you get other messes. You > can no longer simply swap out pages behind the guest's back, copy on > write or combine identical pages or other tricks without synchronizing > with the guest. > > These are good points. > It can be done, Maybe we'll get something from the s390 port. > but it's not clear to me that verifying PTEs is that > much faster than shadowing them if you're prepared to be that invasive > in the guest anyway. > This can probably be measured on Xen which can switch from direct to shadow mode on runtime. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic. ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/