Plumb through vfio_device_get_feature to the vfio-user server. Note that
we translate EINVAL into ENOTTY, as the existing generic vfio code is
expecting the latter to mean "unsupported".

Signed-off-by: John Levon <john.levon@nutanix.com>
---
 hw/vfio-user/protocol.h | 12 ++++++++++++
 hw/vfio-user/device.c   | 39 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+)

diff --git a/hw/vfio-user/protocol.h b/hw/vfio-user/protocol.h
index 3249a4a6b6..2a0c31e7c5 100644
--- a/hw/vfio-user/protocol.h
+++ b/hw/vfio-user/protocol.h
@@ -40,6 +40,7 @@ enum vfio_user_command {
     VFIO_USER_DEVICE_RESET              = 13,
     VFIO_USER_DIRTY_PAGES               = 14,
     VFIO_USER_REGION_WRITE_MULTI        = 15,
+    VFIO_USER_DEVICE_FEATURE            = 16,
     VFIO_USER_MAX,
 };
 
@@ -239,4 +240,15 @@ typedef struct {
     VFIOUserWROne wrs[VFIO_USER_MULTI_MAX];
 } VFIOUserWRMulti;
 
+/*
+ * VFIO_USER_DEVICE_FEATURE
+ * imported from struct vfio_device_feature
+ */
+typedef struct {
+    VFIOUserHdr hdr;
+    uint32_t argsz;
+    uint32_t flags;
+    char data[];
+} VFIOUserDeviceFeature;
+
 #endif /* VFIO_USER_PROTOCOL_H */
diff --git a/hw/vfio-user/device.c b/hw/vfio-user/device.c
index 64ef35b320..6910d183d7 100644
--- a/hw/vfio-user/device.c
+++ b/hw/vfio-user/device.c
@@ -74,6 +74,44 @@ void vfio_user_device_reset(VFIOUserProxy *proxy)
     }
 }
 
+static int
+vfio_user_device_io_device_feature(VFIODevice *vbasedev,
+                                   struct vfio_device_feature *feature)
+{
+    g_autofree VFIOUserDeviceFeature *msgp = NULL;
+    int size = sizeof(VFIOUserHdr) + feature->argsz;
+    VFIOUserProxy *proxy = vbasedev->proxy;
+    Error *local_err = NULL;
+
+    msgp = g_malloc0(size);
+
+    vfio_user_request_msg(&msgp->hdr, VFIO_USER_DEVICE_FEATURE, size, 0);
+
+    memcpy(&msgp->argsz, &feature->argsz, feature->argsz);
+
+    if (!vfio_user_send_wait(proxy, &msgp->hdr, NULL, size, &local_err)) {
+        error_prepend(&local_err, "%s: ", __func__);
+        error_report_err(local_err);
+        return -EFAULT;
+    }
+
+    if (msgp->hdr.flags & VFIO_USER_ERROR) {
+        /*
+         * Client expects ENOTTY for "not supported", but the protocol may
+         * return EINVAL.
+         */
+        if (msgp->hdr.error_reply == EINVAL) {
+            return -ENOTTY;
+        }
+
+        return -msgp->hdr.error_reply;
+    }
+
+    memcpy(feature, &msgp->argsz, feature->argsz);
+
+    return 0;
+}
+
 static int vfio_user_get_region_info(VFIOUserProxy *proxy,
                                      struct vfio_region_info *info,
                                      VFIOUserFDs *fds)
@@ -432,6 +470,7 @@ static int vfio_user_device_io_region_write(VFIODevice *vbasedev, uint8_t index,
  * Socket-based io_ops
  */
 VFIODeviceIOOps vfio_user_device_io_ops_sock = {
+    .device_feature = vfio_user_device_io_device_feature,
     .get_region_info = vfio_user_device_io_get_region_info,
     .get_irq_info = vfio_user_device_io_get_irq_info,
     .set_irqs = vfio_user_device_io_set_irqs,
-- 
2.43.0