[RFC PATCH v8 15/21] vhost: add vhost_svq_poll

["Eugenio Pérez" <eperezma@redhat.com>]

It allows the Shadow Control VirtQueue to wait the device to use the commands
that restore the net device state after a live migration.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
 hw/virtio/vhost-shadow-virtqueue.h |  1 +
 hw/virtio/vhost-shadow-virtqueue.c | 57 +++++++++++++++++++++++++++---
 2 files changed, 54 insertions(+), 4 deletions(-)

diff --git a/hw/virtio/vhost-shadow-virtqueue.h b/hw/virtio/vhost-shadow-virtqueue.h
index 3c55fe2641..20ca59e9a7 100644
--- a/hw/virtio/vhost-shadow-virtqueue.h
+++ b/hw/virtio/vhost-shadow-virtqueue.h
@@ -124,6 +124,7 @@ bool vhost_svq_valid_features(uint64_t features, Error **errp);
 
 int vhost_svq_inject(VhostShadowVirtqueue *svq, const struct iovec *iov,
                      size_t out_num, size_t in_num);
+ssize_t vhost_svq_poll(VhostShadowVirtqueue *svq);
 void vhost_svq_set_svq_kick_fd(VhostShadowVirtqueue *svq, int svq_kick_fd);
 void vhost_svq_set_svq_call_fd(VhostShadowVirtqueue *svq, int call_fd);
 void vhost_svq_get_vring_addr(const VhostShadowVirtqueue *svq,
diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
index c535c99905..831ffb71e5 100644
--- a/hw/virtio/vhost-shadow-virtqueue.c
+++ b/hw/virtio/vhost-shadow-virtqueue.c
@@ -10,6 +10,8 @@
 #include "qemu/osdep.h"
 #include "hw/virtio/vhost-shadow-virtqueue.h"
 
+#include <glib/gpoll.h>
+
 #include "qemu/error-report.h"
 #include "qapi/error.h"
 #include "qemu/main-loop.h"
@@ -583,10 +585,11 @@ static bool vhost_svq_unmap_elem(VhostShadowVirtqueue *svq, SVQElement *svq_elem
     return true;
 }
 
-static void vhost_svq_flush(VhostShadowVirtqueue *svq,
-                            bool check_for_avail_queue)
+static size_t vhost_svq_flush(VhostShadowVirtqueue *svq,
+                              bool check_for_avail_queue)
 {
     VirtQueue *vq = svq->vq;
+    size_t ret = 0;
 
     /* Forward as many used buffers as possible. */
     do {
@@ -604,7 +607,7 @@ static void vhost_svq_flush(VhostShadowVirtqueue *svq,
             if (svq->copy_descs) {
                 bool ok = vhost_svq_unmap_elem(svq, svq_elem, len, true);
                 if (unlikely(!ok)) {
-                    return;
+                    return ret;
                 }
             }
 
@@ -621,10 +624,12 @@ static void vhost_svq_flush(VhostShadowVirtqueue *svq,
                         i, svq->vring.num);
                     virtqueue_fill(vq, elem, len, i);
                     virtqueue_flush(vq, i);
-                    return;
+                    return ret + 1;
                 }
                 virtqueue_fill(vq, elem, len, i++);
             }
+
+            ret++;
         }
 
         if (i > 0) {
@@ -640,6 +645,50 @@ static void vhost_svq_flush(VhostShadowVirtqueue *svq,
             vhost_handle_guest_kick(svq);
         }
     } while (!vhost_svq_enable_notification(svq));
+
+    return ret;
+}
+
+/**
+ * Poll the SVQ for device used buffers.
+ *
+ * This function race with main event loop SVQ polling, so extra
+ * syncthronization is needed.
+ *
+ * Return the number of descriptors read from the device.
+ */
+ssize_t vhost_svq_poll(VhostShadowVirtqueue *svq)
+{
+    int fd = event_notifier_get_fd(&svq->hdev_call);
+    GPollFD poll_fd = {
+        .fd = fd,
+        .events = G_IO_IN,
+    };
+    assert(fd >= 0);
+    int r = g_poll(&poll_fd, 1, -1);
+
+    if (unlikely(r < 0)) {
+        error_report("Cannot poll device call fd "G_POLLFD_FORMAT": (%d) %s",
+                     poll_fd.fd, errno, g_strerror(errno));
+        return -errno;
+    }
+
+    if (r == 0) {
+        return 0;
+    }
+
+    if (unlikely(poll_fd.revents & ~(G_IO_IN))) {
+        error_report(
+            "Error polling device call fd "G_POLLFD_FORMAT": revents=%d",
+            poll_fd.fd, poll_fd.revents);
+        return -1;
+    }
+
+    /*
+     * Max return value of vhost_svq_flush is (uint16_t)-1, so it's safe to
+     * convert to ssize_t.
+     */
+    return vhost_svq_flush(svq, false);
 }
 
 /**
-- 
2.27.0