From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=58201 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1OGDbk-0002sy-1m for qemu-devel@nongnu.org; Sun, 23 May 2010 12:00:57 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1OGDbi-0002kF-GW for qemu-devel@nongnu.org; Sun, 23 May 2010 12:00:55 -0400 Received: from mx1.redhat.com ([209.132.183.28]:25228) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1OGDbi-0002k2-7Y for qemu-devel@nongnu.org; Sun, 23 May 2010 12:00:54 -0400 Date: Sun, 23 May 2010 18:56:05 +0300 From: "Michael S. Tsirkin" Subject: Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself Message-ID: <20100523155605.GB14733@redhat.com> References: <20100505205814.GA7090@redhat.com> <4BF39C12.7090407@redhat.com> <201005201431.51142.rusty@rustcorp.com.au> <201005201438.17010.rusty@rustcorp.com.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <201005201438.17010.rusty@rustcorp.com.au> List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Rusty Russell Cc: qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org, Avi Kivity , kvm@vger.kernel.org, linux-kernel@vger.kernel.org On Thu, May 20, 2010 at 02:38:16PM +0930, Rusty Russell wrote: > On Thu, 20 May 2010 02:31:50 pm Rusty Russell wrote: > > On Wed, 19 May 2010 05:36:42 pm Avi Kivity wrote: > > > > Note that this is a exclusive->shared->exclusive bounce only, too. > > > > > > > > > > A bounce is a bounce. > > > > I tried to measure this to show that you were wrong, but I was only able > > to show that you're right. How annoying. Test code below. > > This time for sure! The share option does not work on some boxes unless I apply the following: essentially, this adds mb() after each write and before read. It seems to make sense to me: we must update our counter before we wanit for another side. diff --git a/cachebounce.c b/cachebounce.c index 0387027..ebe5a37 100644 --- a/cachebounce.c +++ b/cachebounce.c @@ -77,6 +77,7 @@ int main(int argc, char *argv[]) count++; counter->cacheline1 = count; count++; + __sync_synchronize(); } break; case UNSHARE: @@ -86,6 +87,7 @@ int main(int argc, char *argv[]) count++; counter->cacheline2 = count; count++; + __sync_synchronize(); } break; case LOCKSHARE: @@ -98,6 +100,7 @@ int main(int argc, char *argv[]) break; case LOCKUNSHARE: while (count < MAX_BOUNCES) { + __sync_synchronize(); /* Spin waiting for other side to change it. */ while (counter->cacheline1 != count); __sync_val_compare_and_swap(&counter->cacheline2, count, count+1); @@ -115,6 +118,7 @@ int main(int argc, char *argv[]) count++; counter->cacheline1 = count; count++; + __sync_synchronize(); } break; case UNSHARE: @@ -124,6 +128,7 @@ int main(int argc, char *argv[]) count++; counter->cacheline1 = count; count++; + __sync_synchronize(); } break; case LOCKSHARE: