From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: Paravirt KVM capabilities Date: Wed, 10 Jan 2007 13:42:27 +0200 Message-ID: <45A4D123.5070803@qumranet.com> References: <20070109141916.GA13276@vlad.carfax.org.uk> <45A3A642.1030604@qumranet.com> <1168384852.19646.161.camel@localhost.localdomain> <20070110094750.GA934@elte.hu> <45A4BB74.9010102@redhat.com> <20070110101839.GA6444@elte.hu> <45A4C345.5050404@qumranet.com> <20070110105202.GA13412@elte.hu> <45A4C7ED.8090003@qumranet.com> <20070110111442.GA16867@elte.hu> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org Return-path: To: Ingo Molnar In-Reply-To: <20070110111442.GA16867-X9Un+BFzKDI@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 Ingo Molnar wrote: > * Avi Kivity wrote: > > >> For i386 Xen does not switch cr3 IIRC. [...] >> > > correct. > > >> [...] Perhaps even not for x86_64 if it can use the segment limits >> which AMD re-added (I think it does?) >> > > i'm not sure. Older ones definitely used cr3 switching and on Intel > there's no segment limits on 64-bit AFAIK. > > I'd be surprised if that remained the case for too long. >> I think for i386 Xen does not go through the hypervisor at all: it >> hacks int 0x80 to trap directly to ring 1. So there's still the >> overhead of using int rather than sysenter, but not much more. [...] >> > > that alone is already ~100-200 cycles overhead, it essentially doubles > the null syscall overhead. It matters at millions of syscalls per second > workloads, 100 cycles overhead at 1 million syscalls a second means 5% > performance difference at 2GHz. > > That doesn't strike me as too bad. > and longer-term, the cost of VM exit due to IRQs is expected to get > cheaper. The cost of int $0x80 versus SYSENTER will stay constant, or > will get worse. > Agreed. But I still don't know whether the lines on the graph will cross. -- error compiling committee.c: too many arguments to function ------------------------------------------------------------------------- Take Surveys. Earn Cash. Influence the Future of IT Join SourceForge.net's Techsay panel and you'll get the chance to share your opinions on IT & business topics through brief surveys - and earn cash http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV