From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself Date: Sun, 23 May 2010 18:56:05 +0300 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 Cc: Avi Kivity , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, qemu-devel@nongnu.org To: Rusty Russell Return-path: Content-Disposition: inline In-Reply-To: <201005201438.17010.rusty@rustcorp.com.au> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.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: