The cam_simq_alloc function allocates a SIM queue.

#include <cam/cam_sim.h>
#include <cam/cam_queue.h>

struct cam_devq *
cam_simq_alloc(u_int32_t max_sim_transactions);

Here, max_sim_transactions denotes the size of the SIM queue. Normally, it is calculated like so:

max_sim_transactions = number_of_supported_devices *
    number_of_commands_that_can_be_concurrently_processed_per_device;

The cam_sim_alloc function allocates a SIM (or bus) descriptor.

#include <sys/param.h>
#include <sys/lock.h>
#include <sys/mutex.h>

#include <cam/cam_sim.h>
#include <cam/cam_queue.h>

struct cam_sim *
cam_sim_alloc(sim_action_func sim_action, sim_poll_func sim_poll,
    const char *sim_name, void *softc, u_int32_t unit, struct mtx *mtx,
    int max_dev_transactions, int max_tagged_dev_transactions,
    struct cam_devq *queue);

Because the first six arguments to cam_sim_alloc are fairly obvious—they’re exactly what their name implies—I’ll omit discussing them.

The max_dev_transactions argument specifies the maximum number of concurrent transactions per device. This argument applies only to devices that do not support SCSI Tagged Command Queuing (SCSI TCQ). Generally, max_dev_transactions is always set to 1.

The max_tagged_dev_transactions argument is identical to max_dev_transactions, but it applies only to devices that support SCSI TCQ.

The queue argument expects a pointer to a SIM queue (that is, cam_simq_alloc’s return value).

The xpt_bus_register function registers a SIM with the CAM subsystem.

#include <cam/cam_sim.h>
#include <cam/cam_xpt_sim.h>

int32_t
xpt_bus_register(struct cam_sim *sim, device_t parent, u_int32_t bus)

Here, sim specifies the SIM to register (that is, cam_sim_alloc’s return value) and bus denotes its bus number. The parent argument is currently unused.