From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rusty Russell Subject: Re: [PATCHv2] virtio: use smp_XX barriers on SMP Date: Thu, 28 Jan 2010 10:01:09 +1030 Message-ID: <201001281001.09999.rusty@rustcorp.com.au> References: <20100127224222.GA31545@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20100127224222.GA31545@redhat.com> 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: "Michael S. Tsirkin" Cc: virtualization@lists.linux-foundation.org List-Id: virtualization@lists.linuxfoundation.org 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? But I'm not sure what came out of the discussion: Jeremy? Cheers, Rusty.