Re: [Qemu-devel] virtio-serial-pci very expensive during live migration

[Chris Friesen]

On 05/07/2014 12:39 AM, Paolo Bonzini wrote:
> Il 06/05/2014 22:01, Chris Friesen ha scritto:
> > It seems like the main problem is that we loop over all the queues,
> > calling virtio_pci_set_host_notifier_internal() on each of them.  That
> > in turn calls memory_region_add_eventfd(), which calls
> > memory_region_transaction_commit(), which scans over all the address
> > spaces, which seems to take the vast majority of the time.
>
> Yes, you can wrap the entire loop with memory_region_transaction_begin
> and memory_region_transaction_commit.  Can you try that?

I tried the patch below.  Unfortunately it seems to cause qemu to crash.

That said, I'm on a patched version so it might be good if someone else 
tried it.  If it works for other people it might mean bugs in our other 
patches.

Chris





Index: qemu-1.4.2/hw/virtio-pci.c
===================================================================
--- qemu-1.4.2.orig/hw/virtio-pci.c
+++ qemu-1.4.2/hw/virtio-pci.c
@@ -32,6 +32,7 @@
 #include "virtio-pci.h"
 #include "qemu/range.h"
 #include "virtio-bus.h"
+#include "exec/memory.h"

 /* from Linux's linux/virtio_pci.h */

@@ -209,6 +210,7 @@ static void virtio_pci_start_ioeventfd(V
         return;
     }

+    memory_region_transaction_begin();
     for (n = 0; n < VIRTIO_PCI_QUEUE_MAX; n++) {
         if (!virtio_queue_get_num(proxy->vdev, n)) {
             continue;
@@ -219,10 +221,12 @@ static void virtio_pci_start_ioeventfd(V
             goto assign_error;
         }
     }
+    memory_region_transaction_commit();
     proxy->ioeventfd_started = true;
     return;

 assign_error:
+    memory_region_transaction_commit();
     while (--n >= 0) {
         if (!virtio_queue_get_num(proxy->vdev, n)) {
             continue;
@@ -244,6 +248,7 @@ static void virtio_pci_stop_ioeventfd(Vi
         return;
     }

+    memory_region_transaction_begin();
     for (n = 0; n < VIRTIO_PCI_QUEUE_MAX; n++) {
         if (!virtio_queue_get_num(proxy->vdev, n)) {
             continue;
@@ -252,6 +257,7 @@ static void virtio_pci_stop_ioeventfd(Vi
         r = virtio_pci_set_host_notifier_internal(proxy, n, false, false);
         assert(r >= 0);
     }
+    memory_region_transaction_commit();
     proxy->ioeventfd_started = false;
 }