From mboxrd@z Thu Jan 1 00:00:00 1970 From: Martin Schwidefsky Subject: Re: [patch 6/6] Guest page hinting: s390 support. Date: Thu, 13 Mar 2008 10:36:08 +0100 Message-ID: <1205400968.26537.25.camel@localhost> References: <20080312132132.520833247@de.ibm.com> <20080312132704.474209626@de.ibm.com> <47D802A2.1030406@goop.org> <1205339285.8851.13.camel@localhost> <47D8085E.9030701@goop.org> <1205341164.8851.44.camel@localhost> <47D81771.5070400@goop.org> <47D8373C.40105@codemonkey.ws> <47D840E3.5090902@goop.org> Reply-To: schwidefsky@de.ibm.com Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <47D840E3.5090902@goop.org> Sender: linux-kernel-owner@vger.kernel.org To: Jeremy Fitzhardinge Cc: Anthony Liguori , akpm@osdl.org, linux-s390@vger.kernel.org, frankeh@watson.ibm.com, nickpiggin@yahoo.com.au, linux-kernel@vger.kernel.org, virtualization@lists.osdl.org, hugh@veritas.com List-Id: virtualization@lists.linuxfoundation.org On Wed, 2008-03-12 at 13:45 -0700, Jeremy Fitzhardinge wrote: > Vp should never happen, since you'd never preserve a V page. And surely > it would be Pr -> Sr, since the hypervisor wouldn't push the page to > backing store when you change the client state. Vp does not happen in the current implementation. But it actually may be useful. z/VM has multiple layers of paging, the first goes to expanded storage which is very fast. If you make the page Vz and the guests needs it you have to do a standard Linux I/O to get retrieve the page. This can be slower than a read and a write to expanded storage. > > Do the host states even really need visibility to the guest at all? It > > may be useful for the guest to be able to distinguish between Ur and Uz > > but it doesn't seem necessary. > > Well, you implicitly see the hypervisor state. If you touch a [UV]z > page then you get a fault telling you that the page has been taken away > from you (I think). And it would definitely help with debugging (seems > likely there's lots of scope for race conditions if you prematurely tell > the hypervisor you don't need the page any more...). You get an addressing exception if you touch a Uz page. This indicates a BUG in the Linux code because this is a use after free. If the guests touches a Vz page you get a discard fault. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin.