CODEWITHSUNDEEP
assemblyintelinterruptlauterbach
Cumatru
Cumatru

Reputation: 725

Check if specific interrupt is enabled ( IPI )

I've read that EFLAGS has IF where it can store if interrupts are generally enabled, but i need to know if IPI ( inter-process interrupt ) is enabled.

I only have physical access to the CPU registers and data via Lauterbach debugger, i cannot access kernel or stuff like that.

Upvotes: 1

Views: 956

Answers (1)

Margaret Bloom
Margaret Bloom

Reputation: 44046

The term IPI designates a category of interrupts, specifically: NMI, SMI, SIPI, INIT and Fixed interrupts.
Of these, the NMI, SMI, SIPI and INIT cannot be masked without disabling the LAPIC globally.

The NMI (Non-maskable Interrupt) was designed to be non-maskable from the beginning, as the name implies.
There is a hack to mask it: setting the bit7 of port 70h.
However, this is a hardware trick, it ties the #NMI (today it is the LINT1 but this is configurable) high.
The IPIs are sent through a software message so this trick won't work.

The SMI (System Management Interrupt) is used to enter the SMM (see previous link), a mode designed to be as trasparent to the software as possible.
It is not maskable, for what the software is concerned, it doesn't exist.

The INIT and SIPI (Startup-IPI)1 interrupts are used to reset and wake up a CPU.
They cannot be masked by design (the BIOS usually put the APs, Application Processors, to sleep with a cli / hlt sequence).

The Fixed interrupts can be masked with the IF flag or when a higher priority fixed interrupt ISR is executing (just like the legacy IRQs with the PIC).

It's possible to mask only the Fixed interrupts, something that may actually corresponds to the common understanding of the term IPI, by soft disabling the APIC.
This can be done by clearing bit8 of the Spurious Interrupt Vector Register at offset 0f0h from the LAPIC base (the LAPIC base is set in the IA32_APIC_BASE MSR, its address is 1bh).
Of course, clearing IF will also do.

Alternatively it's possible to disable the LAPIC entirely (it won't respond to any IPI) by clearing bit11 of the IA32_APIC_BASE MSR.

To check if the IPIs are enable you have to check the IF flag, the Spurious Interrupt Vector Register and the IA32_APIC_BASE MSR.

Upvotes: 4

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