From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [RFC] virtio: Support releasing lock during kick Date: Fri, 25 Jun 2010 13:43:17 +0300 Message-ID: <20100625104317.GC16321@redhat.com> References: <1277328242-10685-1-git-send-email-stefanha@linux.vnet.ibm.com> <4C2286BE.40808@codemonkey.ws> <201006251239.23224.rusty@rustcorp.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: virtualization@lists.linux-foundation.org, Stefan Hajnoczi , Anthony Liguori , Stefan Hajnoczi , kvm@vger.kernel.org To: Rusty Russell Return-path: Received: from mx1.redhat.com ([209.132.183.28]:48336 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753912Ab0FYKsX (ORCPT ); Fri, 25 Jun 2010 06:48:23 -0400 Content-Disposition: inline In-Reply-To: <201006251239.23224.rusty@rustcorp.com.au> Sender: kvm-owner@vger.kernel.org List-ID: On Fri, Jun 25, 2010 at 12:39:21PM +0930, Rusty Russell wrote: > On Thu, 24 Jun 2010 03:00:30 pm Stefan Hajnoczi wrote: > > On Wed, Jun 23, 2010 at 11:12 PM, Anthony Liguori wrote: > > > Shouldn't it be possible to just drop the lock before invoking > > > virtqueue_kick() and reacquire it afterwards? There's nothing in that > > > virtqueue_kick() path that the lock is protecting AFAICT. > > > > No, that would lead to a race condition because vq->num_added is > > modified by both virtqueue_add_buf_gfp() and virtqueue_kick(). > > Without a lock held during virtqueue_kick() another vcpu could add > > bufs while vq->num_added is used and cleared by virtqueue_kick(): > > Right, this dovetails with another proposed change (was it Michael?) > where we would update the avail idx inside add_buf, rather than waiting > until kick. This means a barrier inside add_buf, but that's probably > fine. > > If we do that, then we don't need a lock on virtqueue_kick. > > Michael, thoughts? Maybe not even that: I think we could just do virtio_wmb() in add, and keep the mb() in kick. What I'm a bit worried about is contention on the cacheline including index and flags: the more we write to that line, the worse it gets. So need to test performance impact of this change: I didn't find time to do this yet, as I am trying to finalize the used index publishing patches. Any takers? Do we see performance improvement after making kick lockless? > Thanks, > Rusty. > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html