From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH] virtio: use smp_XX barriers Date: Wed, 27 Jan 2010 11:46:32 +0200 Message-ID: <4B600B78.1020505@redhat.com> References: <20100121171055.GA16693@redhat.com> <4B600740.3060806@redhat.com> <20100127093610.GD3476@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20100127093610.GD3476@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 01/27/2010 11:36 AM, Michael S. Tsirkin wrote: > > >> smp_mb() is used for processor-vs-processor ordering, which can't happen >> on UP systems, but for process-vs-device, we must use mb(). >> >> (this shows up if running a UP guest on an SMP host). >> > Currently, yes. But virtio is not a real device. > Here's what I was really trying to improve: rmb() is an lfence on > x86_64, but smp_rmb() is a barrier() and this is really sufficient for > virtio because x86_64 does not reorder memory reads. > > x86_64 can do speculative and reordered reads. > Does this mean such an optimization would need a new macro? > Please no. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic.