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NETISR(9) FreeBSD Kernel Developer's Manual NETISR(9)

Kernel network dispatch service

#include <net/netisr.h>
netisr_register(const struct netisr_handler *nhp);
netisr_unregister(const struct netisr_handler *nhp);
netisr_dispatch(u_int proto, struct mbuf *m);
netisr_dispatch_src(u_int proto, uintptr_t source, struct mbuf *m);
netisr_queue(u_int proto, struct mbuf *m);
netisr_queue_src(u_int proto, uintptr_t source, struct mbuf *m);
netisr_clearqdrops(const struct netisr_handler *nhp);
netisr_getqdrops(const struct netisr_handler *nhp, uint64_t *qdropsp);
netisr_getqlimit(const struct netisr_handler *nhp, u_int *qlimitp);
netisr_setqlimit(const struct netisr_handler *nhp, u_int qlimit);
netisr_default_flow2cpu(u_int flowid);
netisr_get_cpuid(u_int cpunumber);
With optional virtual network stack support enabled via the following kernel compile option:
options VIMAGE

netisr_register_vnet(const struct netisr_handler *nhp);
netisr_unregister_vnet(const struct netisr_handler *nhp);

The netisr kernel interface suite allows device drivers (and other packet sources) to direct packets to protocols for directly dispatched or deferred processing. Protocol registration and work stream statistics may be monitored using netstat(1).

Protocols register and unregister handlers using netisr_register() and netisr_unregister(), and may also manage queue limits and statistics using the netisr_clearqdrops(), netisr_getqdrops(), netisr_getqlimit(), and netisr_setqlimit().
In case of VIMAGE kernels each virtual network stack (vnet), that is not the default base system network stack, calls netisr_register_vnet() and netisr_unregister_vnet() to enable or disable packet processing by the netisr for each protocol. Disabling will also purge any outstanding packet from the protocol queue.
netisr supports multi-processor execution of handlers, and relies on a combination of source ordering and protocol-specific ordering and work-placement policies to decide how to distribute work across one or more worker threads. Registering protocols will declare one of three policies:
netisr should maintain source ordering without advice from the protocol. netisr will ignore any flow IDs present on mbuf headers for the purposes of work placement.
netisr should maintain flow ordering as defined by the mbuf header flow ID field. If the protocol implements nh_m2flow, then netisr will query the protocol in the event that the mbuf doesn't have a flow ID, falling back on source ordering.
netisr will entirely delegate all work placement decisions to the protocol, querying nh_m2cpuid for each packet.
Registration is declared using struct netisr_handler, whose fields are defined as follows:
const char * nh_name
Unique character string name of the protocol, which may be included in sysctl(3) MIB names, so should not contain whitespace.
netisr_handler_t nh_handler
Protocol handler function that will be invoked on each packet received for the protocol.
netisr_m2flow_t nh_m2flow
Optional protocol function to generate a flow ID and set a valid hashtype for packets that enter the netisr with M_HASHTYPE_GET(m) equal to M_HASHTYPE_NONE. Will be used only with NETISR_POLICY_FLOW.
netisr_m2cpuid_t nh_m2cpuid
Protocol function to determine what CPU a packet should be processed on. Will be used only with NETISR_POLICY_CPU.
netisr_drainedcpu_t nh_drainedcpu
Optional callback function that will be invoked when a per-CPU queue was drained. It will never fire for directly dispatched packets. Unless fully understood, this special-purpose function should not be used.
u_int nh_proto
Protocol number used by both protocols to identify themselves to netisr, and by packet sources to select what handler will be used to process packets. A table of supported protocol numbers appears below. For implementation reasons, protocol numbers great than 15 are currently unsupported.
u_int nh_qlimit
The maximum per-CPU queue depth for the protocol; due to internal implementation details, the effective queue depth may be as much as twice this number.
u_int nh_policy
The ordering and work placement policy for the protocol, as described earlier.

Packet sources, such as network interfaces, may request protocol processing using the netisr_dispatch() and netisr_queue() interfaces. Both accept a protocol number and mbuf argument, but while netisr_queue() will always execute the protocol handler asynchronously in a deferred context, netisr_dispatch() will optionally direct dispatch if permitted by global and per-protocol policy.
In order to provide additional load balancing and flow information, packet sources may also specify an opaque source identifier, which in practice might be a network interface number or socket pointer, using the netisr_dispatch_src() and netisr_queue_src() variants.

The follow protocol numbers are currently defined:
IGMPv3 loopback
Routing socket loopback
netstat(1), epair(4)

This manual page and the netisr implementation were written by Robert N. M. Watson.
April 25, 2017 FreeBSD 12.0-RELEASE

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