From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCHv2] virtio: use smp_XX barriers on SMP Date: Thu, 28 Jan 2010 15:37:08 +0200 Message-ID: <20100128133708.GA3776@redhat.com> References: <20100127224222.GA31545@redhat.com> <201001281001.09999.rusty@rustcorp.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <201001281001.09999.rusty@rustcorp.com.au> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Rusty Russell Cc: virtualization@lists.linux-foundation.org List-Id: virtualization@lists.linuxfoundation.org On Thu, Jan 28, 2010 at 10:01:09AM +1030, Rusty Russell wrote: > On Thu, 28 Jan 2010 09:12:23 am Michael S. Tsirkin wrote: > > virtio is communicating with a virtual "device" that actually runs on > > another host processor. Thus SMP barriers can be used to control > > memory access ordering. > > > > Where possible, we should use SMP barriers which are more lightweight than > > mandatory barriers, because mandatory barriers also control MMIO effects on > > accesses through relaxed memory I/O windows (which virtio does not use) > > (compare specifically smp_rmb and rmb on x86_64). > > Xen had a similar issue, in that UP guests running on SMP hosts need to issue > SMP barriers. Is this not also a requirement for virtio? Of course it is. That's why I have ifdef CONFIG_SMP and use mandatory barriers on UP. > But I'm not sure what came out of the discussion: Jeremy? > > Cheers, > Rusty.