From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Subject: Re: Paravirt KVM capabilities Date: Wed, 10 Jan 2007 12:14:42 +0100 Message-ID: <20070110111442.GA16867@elte.hu> 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> 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: Avi Kivity Content-Disposition: inline In-Reply-To: <45A4C7ED.8090003-atKUWr5tajBWk0Htik3J/w@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 * 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 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. 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. Ingo ------------------------------------------------------------------------- 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