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tun/tap: add IFF_BACKPRESSURE flag

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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 <simon.schippers@tu-dortmund.de>
Link: https://patch.msgid.link/20260803183641.96882-2-simon.schippers@tu-dortmund.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Simon Schippers
2026-08-03 20:36:37 +02:00
committed by Jakub Kicinski
parent d4e359b360
commit 485e38995c
2 changed files with 8 additions and 0 deletions
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4
include/uapi/linux/if_tun.h
View File

@@ -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

4
tools/include/uapi/linux/if_tun.h
View File

@@ -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