[PATCH net-next v14 1/5] tun/tap: add IFF_BACKPRESSURE flag

Add the IFF_BACKPRESSURE flag to the UAPI header and to its tools/ copy.
The flag has no effect yet, it is the opt-in switch for the qdisc
backpressure logic added by the following patches.

It is added to TUN_FEATURES only in the last patch of the series, once the
implementation is complete. Until then TUNSETIFF silently masks it off, as
it does for any flag outside TUN_FEATURES.

Keeping the flag and its users in separate patches would either leave a
window where backpressure is unconditional, or make the opt-in a later
add-on. Adding the flag first lets every following patch be a no-op
unless it is set.

Signed-off-by: Simon Schippers <[email protected]>
---
 include/uapi/linux/if_tun.h       | 4 ++++
 tools/include/uapi/linux/if_tun.h | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/include/uapi/linux/if_tun.h b/include/uapi/linux/if_tun.h
index 79d53c7a1ebd..a0ddc50a7534 100644
--- a/include/uapi/linux/if_tun.h
+++ b/include/uapi/linux/if_tun.h
@@ -69,6 +69,10 @@
 #define IFF_NAPI_FRAGS	0x0020
 /* Used in TUNSETIFF to bring up tun/tap without carrier */
 #define IFF_NO_CARRIER	0x0040
+/* Stop the queue instead of dropping when the internal ring is full, so an
+ * attached qdisc applies backpressure instead of being bypassed.
+ */
+#define IFF_BACKPRESSURE	0x0080
 #define IFF_NO_PI	0x1000
 /* This flag has no real effect */
 #define IFF_ONE_QUEUE	0x2000
diff --git a/tools/include/uapi/linux/if_tun.h b/tools/include/uapi/linux/if_tun.h
index 2ec07de1d73b..2c85525704c1 100644
--- a/tools/include/uapi/linux/if_tun.h
+++ b/tools/include/uapi/linux/if_tun.h
@@ -67,6 +67,10 @@
 #define IFF_TAP		0x0002
 #define IFF_NAPI	0x0010
 #define IFF_NAPI_FRAGS	0x0020
+/* Stop the queue instead of dropping when the internal ring is full, so an
+ * attached qdisc applies backpressure instead of being bypassed.
+ */
+#define IFF_BACKPRESSURE	0x0080
 #define IFF_NO_PI	0x1000
 /* This flag has no real effect */
 #define IFF_ONE_QUEUE	0x2000
-- 
2.43.0