The Linux kernel presumes a hidraw device to be "active" as long
as an open handle to its character device exists, and during that time
will actively poll its bus for new messages.

The u2f-passthru device keeps an open handle to the hidraw character device
for its entire lifetime, wasting power and causing its queue to be
clogged with irrelevant packets that were not initiated by the guest.

To mitigate this, use the u2f `start` and `stop` callbacks to
dynamically open the handle when the guest is about to use the
u2f-passthru device (start callback), and close it again whenever
the guest stops using it (stop callback).

Signed-off-by: David Bouman <dbouman03@gmail.com>
Fixes: 299976b050bf (hw/usb: Add U2F key passthru mode)
---
 hw/usb/u2f-passthru.c | 84 +++++++++++++++++++++++++++++++------------
 1 file changed, 61 insertions(+), 23 deletions(-)

diff --git a/hw/usb/u2f-passthru.c b/hw/usb/u2f-passthru.c
index b7025d303d..54062ab4d5 100644
--- a/hw/usb/u2f-passthru.c
+++ b/hw/usb/u2f-passthru.c
@@ -118,7 +118,10 @@ static inline uint16_t packet_init_get_bcnt(
 static void u2f_passthru_reset(U2FPassthruState *key)
 {
     timer_del(&key->timer);
-    qemu_set_fd_handler(key->hidraw_fd, NULL, NULL, key);
+
+    if (key->hidraw_fd >= 0) {
+        qemu_set_fd_handler(key->hidraw_fd, NULL, NULL, key);
+    }
     key->last_transaction_time = 0;
     key->current_transactions_start = 0;
     key->current_transactions_end = 0;
@@ -456,47 +459,79 @@ static int u2f_passthru_open_from_scan(void)
 }
 #endif
 
-static void u2f_passthru_unrealize(U2FKeyState *base)
-{
-    U2FPassthruState *key = PASSTHRU_U2F_KEY(base);
-
-    u2f_passthru_reset(key);
-    qemu_close(key->hidraw_fd);
-}
 
-static void u2f_passthru_realize(U2FKeyState *base, Error **errp)
+static int u2f_passthru_open_hidraw(U2FPassthruState *key, Error** errp)
 {
-    U2FPassthruState *key = PASSTHRU_U2F_KEY(base);
     int fd;
-
     if (key->hidraw == NULL) {
 #ifdef CONFIG_LIBUDEV
         fd = u2f_passthru_open_from_scan();
-        if (fd < 0) {
+
+        if (fd < 0 && errp) {
             error_setg(errp, "%s: Failed to find a U2F USB device",
                        TYPE_U2F_PASSTHRU);
-            return;
         }
 #else
-        error_setg(errp, "%s: Missing hidraw", TYPE_U2F_PASSTHRU);
-        return;
+        if (errp) {
+            error_setg(errp, "%s: Missing hidraw", TYPE_U2F_PASSTHRU);
+        }
+        return -1;
 #endif
     } else {
         fd = qemu_open_old(key->hidraw, O_RDWR);
-        if (fd < 0) {
+
+        if (fd < 0 && errp) {
             error_setg(errp, "%s: Failed to open %s", TYPE_U2F_PASSTHRU,
                        key->hidraw);
-            return;
-        }
 
-        if (!u2f_passthru_is_u2f_device(fd)) {
+        } else if (!u2f_passthru_is_u2f_device(fd)) {
             qemu_close(fd);
-            error_setg(errp, "%s: Passed hidraw does not represent "
-                       "a U2F HID device", TYPE_U2F_PASSTHRU);
-            return;
+            if (errp) {
+                error_setg(errp, "%s: Passed hidraw does not represent "
+                           "a U2F HID device", TYPE_U2F_PASSTHRU);
+            }
+            return -1;
         }
     }
-    key->hidraw_fd = fd;
+
+    return fd;
+}
+
+static bool u2f_passthru_start(U2FKeyState *base)
+{
+    U2FPassthruState *key = PASSTHRU_U2F_KEY(base);
+
+    if (key->hidraw_fd < 0) {
+        key->hidraw_fd = u2f_passthru_open_hidraw(key, NULL);
+    }
+    return key->hidraw_fd >= 0;
+}
+
+static void u2f_passthru_stop(U2FKeyState *base)
+{
+    U2FPassthruState *key = PASSTHRU_U2F_KEY(base);
+
+    u2f_passthru_reset(key);
+    if (key->hidraw_fd >= 0) {
+        qemu_close(key->hidraw_fd);
+        key->hidraw_fd = -1;
+    }
+}
+
+static void u2f_passthru_unrealize(U2FKeyState *base)
+{
+    u2f_passthru_stop(base);
+}
+static void u2f_passthru_realize(U2FKeyState *base, Error **errp)
+{
+    U2FPassthruState *key = PASSTHRU_U2F_KEY(base);
+    int fd = u2f_passthru_open_hidraw(key, errp);
+
+    /* we only open the fd to error at start */
+    if (fd >= 0) {
+        qemu_close(fd);
+    }
+    key->hidraw_fd = -1;
     u2f_passthru_reset(key);
 }
 
@@ -531,6 +566,9 @@ static void u2f_passthru_class_init(ObjectClass *klass, void *data)
     kc->realize = u2f_passthru_realize;
     kc->unrealize = u2f_passthru_unrealize;
     kc->recv_from_guest = u2f_passthru_recv_from_guest;
+    kc->start = u2f_passthru_start;
+    kc->stop  = u2f_passthru_stop;
+
     dc->desc = "QEMU U2F passthrough key";
     dc->vmsd = &u2f_passthru_vmstate;
     device_class_set_props(dc, u2f_passthru_properties);
-- 
2.34.1