From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH/RFC 2/2] s390 virtualization interface. Date: Wed, 02 May 2007 07:46:41 +0300 Message-ID: <463817B1.4060900@qumranet.com> References: <1177681235.5770.22.camel@cotte.boeblingen.de.ibm.com> <200704271853.41523.arnd@arndb.de> <20070429080039.GA8332@osiris.ibm.com> <200705012312.11692.arnd@arndb.de> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org, cborntra-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org, schwidefsky-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org To: Arnd Bergmann Return-path: In-reply-to: <200705012312.11692.arnd-r2nGTMty4D4@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 Arnd Bergmann wrote: > On Sunday 29 April 2007, Heiko Carstens wrote: > > >>> Is this data structure extensible? If it is, you probably need >>> some sort of versioning information to make sure that user space >>> doesn't rely on fields that the kernel doesn't know about. >>> >> I don't think we can put in some versioning information here. If >> the kernel decides to increase the version then old userspace >> code would break? >> We rather need some mechanism so userpace can ask the kernel >> "do you support feature x?" and dependent on the answer some >> fields are used or unused. >> > > You could do it the way that ext2 handles compatible and incompatible > features in the on-disk layout: > > Assign a number of bits in the read-only part of the mapping to flags > that the user application can test. A bit in the compatible range mean > that a feature is available to the user application if it wants to > use it. A bit in the incompatible range means that the user space needs > to understand how to use a feature in order to run correctly. > The current ioctl() interface has a feature test call, basically you send down a feature number get see if it's supported or not. Fairly similar to feature bit except it isn't limited in the number of bits. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic. ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/