From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=53119 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1OBv7T-0005t8-Rh for qemu-devel@nongnu.org; Tue, 11 May 2010 15:27:56 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1OBv7S-0008PD-5S for qemu-devel@nongnu.org; Tue, 11 May 2010 15:27:55 -0400 Received: from mx1.redhat.com ([209.132.183.28]:17668) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1OBv7R-0008P2-Ri for qemu-devel@nongnu.org; Tue, 11 May 2010 15:27:54 -0400 Message-ID: <4BE9AF9A.8080005@redhat.com> Date: Tue, 11 May 2010 22:27:22 +0300 From: Avi Kivity MIME-Version: 1.0 Subject: Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself References: <20100505205814.GA7090@redhat.com> <4BE29320.5090506@redhat.com> <201005071253.53393.rusty@rustcorp.com.au> In-Reply-To: <201005071253.53393.rusty@rustcorp.com.au> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Rusty Russell Cc: qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, "Michael S. Tsirkin" On 05/07/2010 06:23 AM, Rusty Russell wrote: > 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. > If one peer writes the tail of the available ring, while the other reads last_used_idx, it's a false bounce, no? Having things on the same cacheline is only worthwhile if they are accessed at the same time. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic.