[Qemu-devel] [PATCH 1/2] virtio: Add backend feature testing functionnality

[Maxime Coquelin]

This patch adds virtio_test_backend_feature() function to
enable checking a backend feature before the negociation
takes place.

It may be used, for example, to check whether the backend
supports VIRTIO_F_VERSION_1 before enabling modern
capabilities.

Cc: Marcel Apfelbaum <address@hidden>
Cc: Michael S. Tsirkin <address@hidden>
Cc: address@hidden
Signed-off-by: Maxime Coquelin <address@hidden>
---
 hw/virtio/virtio.c         | 14 ++++++++++++++
 include/hw/virtio/virtio.h |  2 ++
 2 files changed, 16 insertions(+)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 74c085c..7ab91a1 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -1481,6 +1481,20 @@ void virtio_vmstate_save(QEMUFile *f, void *opaque, 
size_t size)
     virtio_save(VIRTIO_DEVICE(opaque), f);
 }
 
+bool virtio_test_backend_feature(VirtIODevice *vdev,
+                                 unsigned int fbit, Error **errp)
+{
+    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
+    uint64_t feature;
+
+    virtio_add_feature(&feature, fbit);
+
+    assert(k->get_features != NULL);
+    feature = k->get_features(vdev, feature, errp);
+
+    return virtio_has_feature(feature, fbit);
+}
+
 static int virtio_set_features_nocheck(VirtIODevice *vdev, uint64_t val)
 {
     VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index d2490c1..5fb74c8 100644
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -235,6 +235,8 @@ int virtio_set_status(VirtIODevice *vdev, uint8_t val);
 void virtio_reset(void *opaque);
 void virtio_update_irq(VirtIODevice *vdev);
 int virtio_set_features(VirtIODevice *vdev, uint64_t val);
+bool virtio_test_backend_feature(VirtIODevice *vdev,
+                                 unsigned int fbit, Error **errp);
 
 /* Base devices.  */
 typedef struct VirtIOBlkConf VirtIOBlkConf;
-- 
2.7.4