/* User-level thread control block */
struct utcb {
	u32 mr[MR_TOTAL];	/* MRs that are mapped to real registers */
	u32 saved_tag;		/* Saved tag field for stacked ipcs */
	u32 saved_sender;	/* Saved sender field for stacked ipcs */
	u32 mr_rest[MR_REST];	/* Complete the utcb for up to 64 words */
};

• UTCB is a memory block designated as thread local storage that also serves as storage space for per-thread message registers transferred during an IPC between two threads.

• UTCB address is a virtual address that is unique for each thread available on the system.
• UTCB address is discovered at run-time by reading the Kernel Interface Page.
• UTCB address pool is predefined by the microkernel, but each individual address is allocated by the pagers.
• UTCB stores message registers that are transferred between threads during an IPC.
• Depending on whether the IPC is a send or a receive, the message register fields are either transferred to other threads, or overwritten by message registers of other threads. For details please refer to the L4_IPC system call reference page.
• UTCB may contain thread-local information. Currently on the ARM architecture, the only fields on the UTCB that are not message registers are the saved_tag and saved_sender fields, which are used for saving the context of an earlier IPC if a new IPC is going to be made before finishing the first one in a stacked fashion.
• The UTCB structure is subject to change. New fields may be reserved on the UTCB as needed.

 
/* For obtaining the UTCB address */
static inline struct utcb *l4_get_utcb()
{
 	/*
	 * By double dereferencing, we get the private TLS (aka UTCB). First
	 * reference is to the KIP's utcb offset, second is to the utcb itself,
	 * to which the KIP's utcb reference had been updated during context
	 * switch.
	 */
	return *kip_utcb_ref;
}
 
 
/* Functions to read/write utcb registers */
 
/* Reads the message register at the given offset */
static inline unsigned int read_mr(int offset)
{
	if (offset < MR_TOTAL)
		return l4_get_utcb()->mr[offset];
	else
		return l4_get_utcb()->mr_rest[offset - MR_TOTAL];
}
 
/* Writes the given value at the given message register offset */
static inline void write_mr(unsigned int offset, unsigned int val)
{
	if (offset < MR_TOTAL)
		l4_get_utcb()->mr[offset] = val;
	else
		l4_get_utcb()->mr_rest[offset - MR_TOTAL] = val;
}

The below functions are defined for stacked ipc where a new IPC is initiated before finishing the first one. In order to achieve this, important IPC context is saved onto the UTCB in the designated IPC context fields. The saved context includes information such as the sender id and sender tag. Please refer to the L4_IPC system call reference page for further information.

/*
 * If we're about to do another ipc, this saves the last ipc's
 * parameters such as the sender and tag information.
 * Any previously saved data in save slots are destroyed.
 */
static inline void l4_save_ipcregs(void)
{
	l4_get_utcb()->saved_sender = l4_get_sender();
	l4_get_utcb()->saved_tag = l4_get_tag();
}
 
static inline void l4_restore_ipcregs(void)
{
	l4_set_tag(l4_get_utcb()->saved_tag);
	l4_set_sender(l4_get_utcb()->saved_sender);
}

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