From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753949Ab0EGDYB (ORCPT ); Thu, 6 May 2010 23:24:01 -0400 Received: from ozlabs.org ([203.10.76.45]:51028 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753654Ab0EGDYA (ORCPT ); Thu, 6 May 2010 23:24:00 -0400 From: Rusty Russell To: Avi Kivity Subject: Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into =?iso-8859-1?q?ring=09itself?= Date: Fri, 7 May 2010 12:53:52 +0930 User-Agent: KMail/1.13.2 (Linux/2.6.32-21-generic; KDE/4.4.2; i686; ; ) Cc: "Michael S. Tsirkin" , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, qemu-devel@nongnu.org References: <20100505205814.GA7090@redhat.com> <4BE29320.5090506@redhat.com> In-Reply-To: <4BE29320.5090506@redhat.com> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Message-Id: <201005071253.53393.rusty@rustcorp.com.au> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 6 May 2010 07:30:00 pm Avi Kivity wrote: > On 05/05/2010 11:58 PM, Michael S. Tsirkin wrote: > > + /* We publish the last-seen used index at the end of the available ring. > > + * It is at the end for backwards compatibility. */ > > + vr->last_used_idx =&(vr)->avail->ring[num]; > > + /* Verify that last used index does not spill over the used ring. */ > > + BUG_ON((void *)vr->last_used_idx + > > + sizeof *vr->last_used_idx> (void *)vr->used); > > } > > > > Shouldn't this be on its own cache line? It's next to the available ring; because that's where the guest publishes its data. That whole page is guest-write, host-read. Putting it on a cacheline by itself would be a slight pessimization; the host cpu would have to get the last_used_idx cacheline and the avail descriptor cacheline every time. This way, they are sometimes the same cacheline. Hope that clarifies, Rusty.