From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [patch uq/master 2/2] kvm-all.c: define smp_wmb and use it for coalesced mmio Date: Mon, 22 Feb 2010 16:57:29 +0200 Message-ID: <4B829B59.3020803@redhat.com> References: <20100222135906.347393434@amt.cnet> <20100222140210.130087300@amt.cnet> <4B829364.7020300@redhat.com> <20100222144541.GC18992@amt.cnet> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org, qemu-devel@nongnu.org, "Michael S. Tsirkin" To: Marcelo Tosatti Return-path: Received: from mx1.redhat.com ([209.132.183.28]:10167 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753265Ab0BVO5d (ORCPT ); Mon, 22 Feb 2010 09:57:33 -0500 In-Reply-To: <20100222144541.GC18992@amt.cnet> Sender: kvm-owner@vger.kernel.org List-ID: 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. 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. -- error compiling committee.c: too many arguments to function