From mboxrd@z Thu Jan 1 00:00:00 1970 From: Martin Schwidefsky Subject: Re: [patch 0/6] Guest page hinting version 6. Date: Wed, 07 May 2008 09:00:14 +0200 Message-ID: <1210143614.14130.3.camel@localhost> References: <20080312132132.520833247@de.ibm.com> <1210087982.31803.32.camel@localhost> <20080506154612.58cc3070@cuia.bos.redhat.com> <1210132187.17107.40.camel@bodhitayantram.eng.vmware.com> Reply-To: schwidefsky@de.ibm.com Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1210132187.17107.40.camel@bodhitayantram.eng.vmware.com> Sender: linux-kernel-owner@vger.kernel.org To: Zachary Amsden Cc: Rik van Riel , Hugh Dickins , linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, virtualization@lists.osdl.org, akpm@linux-foundation.org, nickpiggin@yahoo.com.au, frankeh@watson.ibm.com, rusty@rustcorp.com.au, jeremy@goop.org, andrea@qumranet.com, clameter@sgi.com, a.p.zijlstra@chello.nl List-Id: virtualization@lists.linuxfoundation.org On Tue, 2008-05-06 at 20:49 -0700, Zachary Amsden wrote: > > I suspect one of the problems is that there are too many state transitions > > to have it implemented with a low overhead on anything but S390, and even > > there you need milicoded instructions to handle things. > > > > If the number of transitions can be reduced, page hinting could be useful > > for KVM, too. > > Spot on Rik, if every transition becomes a hypercall (and a synchronous > one at that), it isn't workable for us. If, on the other hand, you > share the state bits between the guest and hypervisor, you need a giant > (standalone) bit array for per-page state, which is neither convenient > for Linux nor the hypervisor. I believe s390 has an 'instruction' to > migrate the state bits into the hypervisor per-physical-page data > without requiring a hypercall. That is why we invented the millicoded ESSA instruction on s390. We had an emulation of the instruction to test things. It worked but was awfully slow. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin.