[Qemu-devel] [PATCH] virtio-pci: fix host notifiers on bi-endian architectures

[Greg Kurz]

vhost is seriously broken with ppc64le guests, even in the supposedly
supported case where the host is ppc64le and we don't need cross-endian
support.

The TX virtqueue fails to be handled by vhost and falls back to QEMU.
Despite this unexpected scenario where RX is vhost and TX is QEMU, the
guest runs well with reduced upload performances... until you reboot,
migrate, managed save or in fact any operation that causes vhost_net
to be re-started. Network connectivity is then permanantly lost for
the guest.

TX falling back to QEMU is the result of a failed MMIO store emulation
in KVM. Debugging shows that:

kvmppc_emulate_mmio()
 |
 +-> kvmppc_handle_store()
      |
      +-> kvm_io_bus_write()
           |
           +-> __kvm_io_bus_write() returns -EOPNOTSUPP

This happens because no matching device was found:

__kvm_io_bus_write()
 |
 +->kvm_iodevice_write()
     |
     +->ioeventfd_write()
         |
         +->ioeventfd_in_range() returns false for all registered vrings

Extra debugging shows that the TX vring number (16-bit) is supposed to
be 0x0100 but QEMU passes 0x0001 to KVM... This happens *again* because
QEMU still assumes powerpc is big endian (TARGET_WORDS_BIGENDIAN) by
default.

This patch adds an extra swap in virtio_pci_set_host_notifier_internal()
to negate the one that is done in adjust_endianness(). Since this is not
a hot path and we want to keep virtio-pci.o in common-obj, we don't care
whether the guest is bi-endian or not.

Reported-by: Cédric Le Goater <address@hidden>
Suggested-by: Michael Roth <address@hidden>
Signed-off-by: Greg Kurz <address@hidden>
---

I guess it is also a fix for virtio-1 but I didn't check.

 hw/virtio/virtio-pci.c |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index e7baf7b..62b04c9 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -133,6 +133,11 @@ static int virtio_pci_load_queue(DeviceState *d, int n, 
QEMUFile *f)
     return 0;
 }
 
+static uint16_t cpu_to_host_notifier16(VirtIODevice *vdev, uint16_t val)
+{
+    return virtio_is_big_endian(vdev) ? val : bswap16(val);
+}
+
 static int virtio_pci_set_host_notifier_internal(VirtIOPCIProxy *proxy,
                                                  int n, bool assign, bool 
set_handler)
 {
@@ -150,10 +155,12 @@ static int 
virtio_pci_set_host_notifier_internal(VirtIOPCIProxy *proxy,
         }
         virtio_queue_set_host_notifier_fd_handler(vq, true, set_handler);
         memory_region_add_eventfd(&proxy->bar, VIRTIO_PCI_QUEUE_NOTIFY, 2,
-                                  true, n, notifier);
+                                  true, cpu_to_host_notifier16(vdev, n),
+                                  notifier);
     } else {
         memory_region_del_eventfd(&proxy->bar, VIRTIO_PCI_QUEUE_NOTIFY, 2,
-                                  true, n, notifier);
+                                  true, cpu_to_host_notifier16(vdev, n),
+                                  notifier);
         virtio_queue_set_host_notifier_fd_handler(vq, false, false);
         event_notifier_cleanup(notifier);
     }