From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dor Laor Subject: Re: [PATCH 2/6] virtio block driver for QEMU (v2) Date: Mon, 12 Nov 2007 14:21:03 +0200 Message-ID: <4738452F.8070502@qumranet.com> References: <11947512401155-git-send-email-aliguori@us.ibm.com> <11947512432057-git-send-email-aliguori@us.ibm.com> <47373ADF.3000607@qumranet.com> <47375731.7020007@us.ibm.com> Reply-To: dor.laor-atKUWr5tajBWk0Htik3J/w@public.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org, Avi Kivity To: Anthony Liguori Return-path: In-Reply-To: <47375731.7020007-r/Jw6+rmf7HQT0dZR+AlfA@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 Anthony Liguori wrote: > Dor Laor wrote: >>> >> In general I think we need to add another feature or even version >> number ( I know you guys hate it). >> The reason is - Let's say you dont change functionality but change >> the irq protocol (for example the isr won't be zeroed on read), then >> an old >> guest driver wouldn't know it runs on a new host version and will >> have its irq line pulled up. >> So I suggest adding a capability of VIRTIO_ISR_CLEAR_XXX or adding a >> version number. >> Comments? > > I don't think we'll actually change the ISR protocol. I think it's > the best that it can actually be. However, if we do need to change > the ABI for some reason, I think the right thing to do is just use a > new device ID (since it's effectively a new device). > > Regards, > > Anthony Liguori Changing the devid is acceptable and much more easier then backward compatibility support, I prefer it too. Note that there are some disadvantages when changing the devid - For example, if you installed an old device drivers in the guest kernel and after a while you bring the guest down, upgrade the kvm host version and bring the guest back up. If it has a new device id (and since the abi is not maintained the driver won't match VENDOR=virtio; DEVID=*), then the guest won't have a driver for the new device. In that case I think a guest agent can switch to fully virtualized device and afterwards install the driver automatically. This is what we do in our production environment. Regards, Dor ------------------------------------------------------------------------- This SF.net email is sponsored by: Splunk Inc. Still grepping through log files to find problems? Stop. Now Search log events and configuration files using AJAX and a browser. Download your FREE copy of Splunk now >> http://get.splunk.com/