From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rusty Russell Subject: Re: [PATCHv3 1/2] virtio: support layout with avail ring before idx Date: Fri, 4 Jun 2010 20:46:49 +0930 Message-ID: <201006042046.49872.rusty@rustcorp.com.au> References: <201006041204.57973.rusty@rustcorp.com.au> <20100604103543.GA22270@redhat.com> Mime-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Cc: linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, qemu-devel@nongnu.org, Andrew Morton To: "Michael S. Tsirkin" Return-path: In-Reply-To: <20100604103543.GA22270@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Fri, 4 Jun 2010 08:05:43 pm Michael S. Tsirkin wrote: > On Fri, Jun 04, 2010 at 12:04:57PM +0930, Rusty Russell wrote: > > On Wed, 2 Jun 2010 12:17:12 am Michael S. Tsirkin wrote: > > > This adds an (unused) option to put available ring before control (avail > > > index, flags), and adds padding between index and flags. This avoids > > > cache line sharing between control and ring, and also makes it possible > > > to extend avail control without incurring extra cache misses. > > > > > > Signed-off-by: Michael S. Tsirkin > > > > No no no no. 254? You're trying to Morton me![1] > > Hmm, I wonder what will we do if we want a 3rd field on > a separate chacheline. But ok. > > > How's this (untested): > > I think we also want to put flags there as well, > they are used on interrupt path, together with last used index. I'm uncomfortable with moving a field. We haven't done that before and I wonder what will break with old code. Should we instead just abandon the flags field and use last_used only? Or, more radically, put flags == last_used when the feature is on? Thoughts? Rusty.