vhost: Add VhostShadowVirtqueue

Vhost shadow virtqueue (SVQ) is an intermediate jump for virtqueue
notifications and buffers, allowing qemu to track them. While qemu is
forwarding the buffers and virtqueue changes, it is able to commit the
memory it's being dirtied, the same way regular qemu's VirtIO devices
do.

This commit only exposes basic SVQ allocation and free. Next patches of
the series add functionality like notifications and buffers forwarding.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

hw/virtio/meson.build

@@ -11,7 +11,7 @@ softmmu_ss.add(when: 'CONFIG_ALL', if_true: files('vhost-stub.c'))
 
 virtio_ss = ss.source_set()
 virtio_ss.add(files('virtio.c'))
-virtio_ss.add(when: 'CONFIG_VHOST', if_true: files('vhost.c', 'vhost-backend.c'))
+virtio_ss.add(when: 'CONFIG_VHOST', if_true: files('vhost.c', 'vhost-backend.c', 'vhost-shadow-virtqueue.c'))
 virtio_ss.add(when: 'CONFIG_VHOST_USER', if_true: files('vhost-user.c'))
 virtio_ss.add(when: 'CONFIG_VHOST_VDPA', if_true: files('vhost-vdpa.c'))
 virtio_ss.add(when: 'CONFIG_VIRTIO_BALLOON', if_true: files('virtio-balloon.c'))

hw/virtio/vhost-shadow-virtqueue.c

@@ -0,0 +1,62 @@
+/*
+ * vhost shadow virtqueue
+ *
+ * SPDX-FileCopyrightText: Red Hat, Inc. 2021
+ * SPDX-FileContributor: Author: Eugenio Pérez <eperezma@redhat.com>
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#include "qemu/osdep.h"
+#include "hw/virtio/vhost-shadow-virtqueue.h"
+
+#include "qemu/error-report.h"
+
+/**
+ * Creates vhost shadow virtqueue, and instructs the vhost device to use the
+ * shadow methods and file descriptors.
+ *
+ * Returns the new virtqueue or NULL.
+ *
+ * In case of error, reason is reported through error_report.
+ */
+VhostShadowVirtqueue *vhost_svq_new(void)
+{
+    g_autofree VhostShadowVirtqueue *svq = g_new0(VhostShadowVirtqueue, 1);
+    int r;
+
+    r = event_notifier_init(&svq->hdev_kick, 0);
+    if (r != 0) {
+        error_report("Couldn't create kick event notifier: %s (%d)",
+                     g_strerror(errno), errno);
+        goto err_init_hdev_kick;
+    }
+
+    r = event_notifier_init(&svq->hdev_call, 0);
+    if (r != 0) {
+        error_report("Couldn't create call event notifier: %s (%d)",
+                     g_strerror(errno), errno);
+        goto err_init_hdev_call;
+    }
+
+    return g_steal_pointer(&svq);
+
+err_init_hdev_call:
+    event_notifier_cleanup(&svq->hdev_kick);
+
+err_init_hdev_kick:
+    return NULL;
+}
+
+/**
+ * Free the resources of the shadow virtqueue.
+ *
+ * @pvq: gpointer to SVQ so it can be used by autofree functions.
+ */
+void vhost_svq_free(gpointer pvq)
+{
+    VhostShadowVirtqueue *vq = pvq;
+    event_notifier_cleanup(&vq->hdev_kick);
+    event_notifier_cleanup(&vq->hdev_call);
+    g_free(vq);
+}

hw/virtio/vhost-shadow-virtqueue.h

@@ -0,0 +1,28 @@
+/*
+ * vhost shadow virtqueue
+ *
+ * SPDX-FileCopyrightText: Red Hat, Inc. 2021
+ * SPDX-FileContributor: Author: Eugenio Pérez <eperezma@redhat.com>
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#ifndef VHOST_SHADOW_VIRTQUEUE_H
+#define VHOST_SHADOW_VIRTQUEUE_H
+
+#include "qemu/event_notifier.h"
+
+/* Shadow virtqueue to relay notifications */
+typedef struct VhostShadowVirtqueue {
+    /* Shadow kick notifier, sent to vhost */
+    EventNotifier hdev_kick;
+    /* Shadow call notifier, sent to vhost */
+    EventNotifier hdev_call;
+} VhostShadowVirtqueue;
+
+VhostShadowVirtqueue *vhost_svq_new(void);
+
+void vhost_svq_free(gpointer vq);
+G_DEFINE_AUTOPTR_CLEANUP_FUNC(VhostShadowVirtqueue, vhost_svq_free);
+
+#endif