From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1O9NMI-0001Tp-2w for qemu-devel@nongnu.org; Tue, 04 May 2010 15:00:42 -0400 Received: from [140.186.70.92] (port=59087 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1O9NMC-0001PX-Ke for qemu-devel@nongnu.org; Tue, 04 May 2010 15:00:41 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1O9NMA-0000KJ-NY for qemu-devel@nongnu.org; Tue, 04 May 2010 15:00:36 -0400 Received: from mx1.redhat.com ([209.132.183.28]:59432) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1O9NMA-0000Jr-G6 for qemu-devel@nongnu.org; Tue, 04 May 2010 15:00:34 -0400 Date: Tue, 4 May 2010 21:56:18 +0300 From: "Michael S. Tsirkin" Message-ID: <20100504185618.GA29725@redhat.com> References: <20100218222220.GA14847@redhat.com> <20100504185459.GA24998@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100504185459.GA24998@lst.de> Subject: [Qemu-devel] Re: [PATCH] virtio-spec: document block CMD and FLUSH List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Christoph Hellwig Cc: kvm@vger.kernel.org, Rusty Russell , qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org On Tue, May 04, 2010 at 08:54:59PM +0200, Christoph Hellwig wrote: > On Fri, Feb 19, 2010 at 12:22:20AM +0200, Michael S. Tsirkin wrote: > > I took a stub at documenting CMD and FLUSH request types in virtio > > block. Christoph, could you look over this please? > > > > I note that the interface seems full of warts to me, > > this might be a first step to cleaning them. > > The whole virtio-blk interface is full of warts. It has been > extended rather ad-hoc, so that is rather expected. > > > One issue I struggled with especially is how type > > field mixes bits and non-bit values. I ended up > > simply defining all legal values, so that we have > > CMD = 2, CMD_OUT = 3 and so on. > > It's basically a complete mess without much logic behind it. > > > +\change_unchanged > > +the high bit > > +\change_inserted 0 1266497301 > > + (VIRTIO_BLK_T_BARRIER) > > +\change_unchanged > > + indicates that this request acts as a barrier and that all preceeding requests > > + must be complete before this one, and all following requests must not be > > + started until this is complete. > > + > > +\change_inserted 0 1266504385 > > + Note that a barrier does not flush caches in the underlying backend device > > + in host, and thus does not serve as data consistency guarantee. > > + Driver must use FLUSH request to flush the host cache. > > +\change_unchanged > > I'm not sure it's even worth documenting it. I can't see any way to > actually implement safe behaviour with the VIRTIO_BLK_T_BARRIER-style > barriers. lguest seems to still use this. I guess if you have a reliable host, VIRTIO_BLK_T_BARRIER is enough? > Btw, did I mention that .lyx is a a really horrible format to review > diffs for? Plain latex would be a lot better..