From patchwork Fri Jun 4 02:34:57 2010 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Rusty Russell X-Patchwork-Id: 104199 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by demeter.kernel.org (8.14.3/8.14.3) with ESMTP id o542ZNmZ009371 for ; Fri, 4 Jun 2010 02:35:23 GMT Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757204Ab0FDCfB (ORCPT ); Thu, 3 Jun 2010 22:35:01 -0400 Received: from ozlabs.org ([203.10.76.45]:32933 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756997Ab0FDCfA (ORCPT ); Thu, 3 Jun 2010 22:35:00 -0400 Received: from vivaldi.localnet (ppp121-45-40-179.lns20.adl2.internode.on.net [121.45.40.179]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by ozlabs.org (Postfix) with ESMTPSA id 160F5B7D20; Fri, 4 Jun 2010 12:34:59 +1000 (EST) From: Rusty Russell To: "Michael S. Tsirkin" Subject: Re: [PATCHv3 1/2] virtio: support layout with avail ring before idx Date: Fri, 4 Jun 2010 12:04:57 +0930 User-Agent: KMail/1.13.2 (Linux/2.6.32-22-generic; KDE/4.4.2; i686; ; ) Cc: linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, qemu-devel@nongnu.org, Andrew Morton References: <10a74b58c908bad64ff890c881e2b2de88687f0e.1275403477.git.mst@redhat.com> In-Reply-To: <10a74b58c908bad64ff890c881e2b2de88687f0e.1275403477.git.mst@redhat.com> MIME-Version: 1.0 Message-Id: <201006041204.57973.rusty@rustcorp.com.au> Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org X-Greylist: IP, sender and recipient auto-whitelisted, not delayed by milter-greylist-4.2.3 (demeter.kernel.org [140.211.167.41]); Fri, 04 Jun 2010 02:35:23 +0000 (UTC) diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h --- a/include/linux/virtio_ring.h +++ b/include/linux/virtio_ring.h @@ -74,8 +74,8 @@ struct vring { /* The standard layout for the ring is a continuous chunk of memory which looks * like this. We assume num is a power of 2. * - * struct vring - * { + * struct vring { + * *** The driver writes to this part. * // The actual descriptors (16 bytes each) * struct vring_desc desc[num]; * @@ -84,9 +84,11 @@ struct vring { * __u16 avail_idx; * __u16 available[num]; * - * // Padding to the next align boundary. + * // Padding so used_flags is on the next align boundary. * char pad[]; + * __u16 last_used; // On a cacheline of its own. * + * *** The device writes to this part. * // A ring of used descriptor heads with free-running index. * __u16 used_flags; * __u16 used_idx; @@ -110,6 +112,12 @@ static inline unsigned vring_size(unsign + sizeof(__u16) * 2 + sizeof(struct vring_used_elem) * num; } +/* Last used index sits at the very end of the driver part of the struct */ +static inline __u16 *vring_last_used_idx(const struct vring *vr) +{ + return (__u16 *)vr->used - 1; +} + #ifdef __KERNEL__ #include struct virtio_device;