From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1O9NOM-0003Fl-Mi for qemu-devel@nongnu.org; Tue, 04 May 2010 15:02:50 -0400 Received: from [140.186.70.92] (port=59501 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1O9NOL-0003Dt-EV for qemu-devel@nongnu.org; Tue, 04 May 2010 15:02:50 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1O9NOJ-0000dj-ME for qemu-devel@nongnu.org; Tue, 04 May 2010 15:02:49 -0400 Received: from mx1.redhat.com ([209.132.183.28]:61491) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1O9NOI-0000dZ-Ro for qemu-devel@nongnu.org; Tue, 04 May 2010 15:02:47 -0400 Date: Tue, 4 May 2010 21:58:37 +0300 From: "Michael S. Tsirkin" Message-ID: <20100504185837.GB29725@redhat.com> References: <20100218222220.GA14847@redhat.com> <20100504185459.GA24998@lst.de> <20100504185618.GA29725@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100504185618.GA29725@redhat.com> 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 09:56:18PM +0300, Michael S. Tsirkin wrote: > 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. Sorry, it doesn't. No idea why I thought it does. > 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..