From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Subject: Re: [RFT] mmu optimizations branch Date: Wed, 3 Jan 2007 11:02:35 +0100 Message-ID: <20070103100235.GA17168@elte.hu> References: <4598E33B.608@qumranet.com> <20070102161117.GA3306@elte.hu> <459A8909.7020600@qumranet.com> <20070103022241.GA13840@elte.hu> <459B6A15.4010208@qumranet.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm-devel Return-path: To: Avi Kivity Content-Disposition: inline In-Reply-To: <459B6A15.4010208-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: > >actually, the VM entry+exit cost on this CPU is around 3-4 > >microseconds, so it's still 2 VM exits per context switch. > > > >I debugged this a bit, and what happens is that when Linux does a > >task-switch it does a cr3 load /and/ a write (look at __flush_tlb()) > >- and both are causing a vm exit! > > > >I have started paravirtualizing the Linux kernel for KVM. I have > >eliminated the cr3 load from the Linux kernel via paravirtualization > >and that way lat_ctx shows a ~5-6 usecs context-switch cost. That's > >pretty good i think, compared to the 2-3 usecs of native. I'll send > >patches for this tomorrow. > > Does this really call for parvirtualization? Caching the old cr3 value > will work for native as well. it's already cached (and my changes use that cached value), but it (used to be) faster/smaller to just move from cr3 than to dereference down to the cached pgd pointer, because we used to inline those instructions heavily. But i agree that in the current kernel it's probably worth doing this for native too - i'll cook up a patch for upstream. 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