[Qemu-devel] [PATCH] virtio-pci: Disable virtio-ioeventfd when !CONFIG_IOTHREAD

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

It is not possible to use virtio-ioeventfd when building without an I/O
thread.  We rely on a signal to kick us out of vcpu execution.  Timers
and AIO use SIGALRM and SIGUSR2 respectively.  Unfortunately eventfd
does not support O_ASYNC (SIGIO) so eventfd cannot be used in a signal
driven manner.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
---
 hw/virtio-pci.c |   21 ++++++++++++++++++++-
 1 files changed, 20 insertions(+), 1 deletions(-)

diff --git a/hw/virtio-pci.c b/hw/virtio-pci.c
index d07ff97..e921eda 100644
--- a/hw/virtio-pci.c
+++ b/hw/virtio-pci.c
@@ -640,6 +640,25 @@ static int virtio_pci_set_host_notifier(void *opaque, int n, bool assign)
     return virtio_pci_set_host_notifier_internal(proxy, n, assign);
 }
 
+static bool virtio_pci_can_use_ioeventfd(void)
+{
+    if (!kvm_has_many_ioeventfds()) {
+        return false;
+    }
+
+    /* Use ioeventfd for virtqueue kick only if we have an I/O thread to
+     * perform out-of-line processing.  Otherwise we might as well do
+     * synchronous virtqueue kicks and in fact we have to since eventfd does
+     * not support SIGIO.  Without the I/O thread a signal would be required to
+     * kick the vcpu out of guest code.
+     */
+#ifdef CONFIG_IOTHREAD
+    return true;
+#else
+    return false;
+#endif
+}
+
 static void virtio_pci_vmstate_change(void *opaque, bool running)
 {
     VirtIOPCIProxy *proxy = opaque;
@@ -705,7 +724,7 @@ static void virtio_init_pci(VirtIOPCIProxy *proxy, VirtIODevice *vdev,
     pci_register_bar(&proxy->pci_dev, 0, size, PCI_BASE_ADDRESS_SPACE_IO,
                            virtio_map);
 
-    if (!kvm_has_many_ioeventfds()) {
+    if (!virtio_pci_can_use_ioeventfd()) {
         proxy->flags &= ~VIRTIO_PCI_FLAG_USE_IOEVENTFD;
     }
 
-- 
1.7.2.3