Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself

[Michael S. Tsirkin]

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: