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.
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| @@ -69,6 +69,10 @@ · 68 unmodified lines | ||
| 69 | 69 | #define IFF_NAPI_FRAGS 0x0020 |
| 70 | 70 | /* Used in TUNSETIFF to bring up tun/tap without carrier */ |
| 71 | 71 | #define IFF_NO_CARRIER 0x0040 |
| 72 | +/* Stop the queue instead of dropping when the internal ring is full, so an | |
| 73 | + * attached qdisc applies backpressure instead of being bypassed. | |
| 74 | + */ | |
| 75 | +#define IFF_BACKPRESSURE 0x0080 | |
| 72 | 76 | #define IFF_NO_PI 0x1000 |
| 73 | 77 | /* This flag has no real effect */ |
| 74 | 78 | #define IFF_ONE_QUEUE 0x2000 |