From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [patch uq/master 2/2] kvm-all.c: define smp_wmb and use it for coalesced mmio Date: Mon, 22 Feb 2010 16:57:39 +0200 Message-ID: <20100222145739.GC20580@redhat.com> References: <20100222135906.347393434@amt.cnet> <20100222140210.130087300@amt.cnet> <4B829364.7020300@redhat.com> <20100222144541.GC18992@amt.cnet> <4B829B59.3020803@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Marcelo Tosatti , kvm@vger.kernel.org, qemu-devel@nongnu.org To: Avi Kivity Return-path: Received: from mx1.redhat.com ([209.132.183.28]:53719 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753720Ab0BVPA4 (ORCPT ); Mon, 22 Feb 2010 10:00:56 -0500 Content-Disposition: inline In-Reply-To: <4B829B59.3020803@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Mon, Feb 22, 2010 at 04:57:29PM +0200, Avi Kivity wrote: > On 02/22/2010 04:45 PM, Marcelo Tosatti wrote: >> On Mon, Feb 22, 2010 at 04:23:32PM +0200, Avi Kivity wrote: >> >>> On 02/22/2010 03:59 PM, Marcelo Tosatti wrote: >>> >>>> Cc: "Michael S. Tsirkin" >>>> Signed-off-by: Marcelo Tosatti >>>> >>>> Index: qemu/kvm-all.c >>>> =================================================================== >>>> --- qemu.orig/kvm-all.c >>>> +++ qemu/kvm-all.c >>>> @@ -718,6 +718,9 @@ static int kvm_handle_io(uint16_t port, >>>> return 1; >>>> } >>>> >>>> +/* FIXME: arch dependant, x86 version */ >>>> +#define smp_wmb() asm volatile("" ::: "memory") >>>> + >>>> >>> sfence? >>> >> There is no need (for this case). Older read cannot be reordered with >> write, writes are not reordered with other writes, writes by a single >> processor are observed in the same order by all processors. >> > > Well, Linux does use sfence. At least on 64 bit it doesnt. > Perhaps it's only needed for WC writes > (movnti and friends), but better be careful here. > >>> what about other arches? >>> >> They need to be fixed? PPC needs an instruction apparently. >> >> Is there any objection to including this patch? >> > > I imagine all arches need an instruction. For reads as well. > > Note, gcc has a __sync_synchronize() builtin that compiles to mfence on > x86. We might use that as a baseline for both rmb and wmb, and let each > arch override it incrementally. This it what my patch did. Note it only works well for recent gcc. > -- > error compiling committee.c: too many arguments to function