Secure Partition Interrupt Handling

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Secure Partitions that implement peripheral drivers may need to use interrupts to efficiently manage PE utilization.

Partition implementation

IRQ lines declared in manifest files for Secure Partitions are assigned IRQ signals as described in PSA Firmware Framework.
Interrupt handling is implemented as interrupt service routines (Partition ISR) that are executed in the partition context.
Partitions that are owners of IRQ must define interrupt service routines for them.

manifest file IRQ declaration example

"irqs": [
  {
    "line_num": 17,
    "signal": "RTC"
  },
  {
    "line_name": "UART1_IRQ",
    "signal": "UART1"
  }
]

Partition ISR function

The symbol name of the ISR implemented for a given interrupt must be manifest[‘irqs’][idx].signal attribute post-fixed with “_isr”.

Partition ISR has the signature of a regular interrupt service routine, e.g.:

void UART1_isr(void);

SPM behaviour

In the vector table, each interrupt that has an associated partition ISR is assigned to an SPM interrupt handler (SPM ISR) that delegates handling to a Partition ISR.
Each partition is allocated an asserted signal mask. Every IRQ associated with the partition is assigned a position in the signal mask.

When a secure hardware interrupt is asserted, the SPM:

• Acknowledges the interrupt and masks the hardware interrupt line.
• Identifies the Secure Partition which has registered the interrupt (in the manifest). This identification can happen either
  • using a runtime lookup or
  • by registering different instances of the SPM ISR for each interrupt, so the runtime lookup is avoided both for the partition Id and the ISR function location.
• Sets up execution environment for the secure partition that has registered the interrupt.
• Asserts the IRQ signal for the interrupt in the partition's signal mask.
• Execute Partition ISR. If the partition’s stack is active at the time of pre-emption by the interrupt (i.e. the Secure Partition is not in idle state), the Partition ISR stack frame will be amended to that. If the Secure Partition had been idle, a stack frame will be reserved for the duration of the Partition ISR execution. The Secure Partition state is changed to running for the duration of the Partition ISR.

When the Secure Partition ISR returns, execution is returned to the context pre-empted by the IRQ.

Implementation notes

Interrupts can pre-empt NSPE or secure service execution. Pre-emption of an interrupt is only possible by an interrupt of a higher priority, ensuring deterministic nesting/un-nesting of stack frames.

API functions

psa_wait()

A call to psa_wait() from a secure service yields execution to the framework and becomes blocked waiting for the assertion of a signal. In the meantime, SPM will schedule other contexts that are ready to run. The client remains blocked until the service function returns.

If an IRQ signal matching one in signal_mask to psa_wait() is asserted, the Secure Partition becomes ready to run. When the scheduler activates the Secure Partition, the IRQ signal(s) that had been asserted are returned by psa_wait(). When the service function completes its execution and returns, control is taken back to client.

Note
The only signals implemented in the current TF-M implementation are interrupt signals.

Signature

psa_signal_t psa_wait(psa_signal_t signal_mask, uint32_t timeout);

Parameters

psa_signal_t signal_mask defines the set of interrupt signals that can resume execution of the secure service.
uint32_t timeout defines timeout for the function, as defined in PSA Firmware Framework 1.0-beta-0 (Chapter 4.3.3).

Return

The return value indicates the signal(s) that triggered the resumption of the service; i.e. If multiple interrupt events have been handled, it will be indicated by the mask value in the return code.

tfm_enable_irq()

A call to tfm_enable_irq() from a secure service enables an irq.

Signature

void tfm_enable_irq(psa_signal_t irq_signal);

Parameters

psa_signal_t irq_signal defines the interrupt signal to be enabled.

Return

void
Success.

Does not return
The call is invalid, one or more of the following are true:

• irq_signal is not an interrupt signal.
• irq_signal indicates more than one signal.

tfm_disable_irq()

A call to tfm_disable_irq() from a secure service disables an irq.

Signature

void tfm_disable_irq(psa_signal_t irq_signal);

Parameters

psa_signal_t irq_signal defines the interrupt signal to be disabled.

Return

void
Success.

Does not return
The call is invalid, one or more of the following are true:

• irq_signal is not an interrupt signal.
• irq_signal indicates more than one signal.

psa_eoi()

A call to psa_eoi() from a secure service function or a Partition ISR informs SPM that an interrupt has been processed. This clears the IRQ signal in the asserted signal mask associated with the partition.

Signature

void psa_eoi(psa_signal_t irq_signal);

Parameters

psa_signal_t irq_signal defines the interrupt signal that has been processed.

Return

void
Success.

Does not return
The call is invalid, one or more of the following are true:

• irq_signal is not an interrupt signal.
• irq_signal indicates more than one signal.
• irq_signal is not currently asserted.