3.1.5 Choosing between LPIs and SPIs

Message-based interrupts can be either LPIs or SPIs.

The decision to use an LPI or SPI for an interrupt can be made by software, and depends on whether there are spare SPIs and if the GIC-600 has ITS support. This can be achieved by either making the peripheral write to a different GIC-600 address, or by changing the address translation for the interrupt write in the SMMU. Changing only the SMMU is possible because the registers for Non-secure message-based interrupts, GICD_SETSPI_NSR and GITS_TRANSLATER, or GICR_SETLPIR for configurations without LPI support, are at the same address offset in different pages.

The following factors can help you to decide which interrupt type is most appropriate:

For more information, see Arm® GICv3 and GICv4 Software Overview.

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