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[Trustzone] interrupt level fast world switch(NWD->SWD) solution?

ray mao
Note: This was originally posted on 11th October 2012 at http://forums.arm.com

Dear ARM Engineer,

I have a question about interrupt level fast word switch for trustzone enabled ecosystem.
For scenario - 1 core is running in one World, interrupt of the other World happen. How to switch the core to the other world quickly.

Solution-1
In the whitepaper prd29-genc-009492c_trustzone_security_whitepaper.pdf
one solution is already mentioned. - monitor route slolution.
but this solution need carefully deal with monitor, NWD OS, SWD OS interrupt related code.

Solution-2
I was checking another solution which one World OS actively do swtich directly while the other world interrupt happen.
From ARM GIC Spec, spurious interrupt ID 1022 & 1023 seems to be possible as the flag.
(Suppose NWD only use IRQ, SWD only use FIQ)
After test,
1022 can help SWD->NWD  for case cpu core in SWD and IRQ happened.
1023 can not help NWD->SWD for case cpu core in NWD and FIQ happened. (As 1023 will also happen in other case)

So is there any extra flag/way can help Solution-2 do NWD->SWD.
Or say Besides Monitro solution, is there any other way can help interrupt level fast word switch NWD->SWD while  FIQ happen.

Thanks!
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  • Arm Employee Badge
    Note: This was originally posted on 11th October 2012 at http://forums.arm.com

    The basic problem with (2) is that it relies on both OSs (NWD and SWD) behaving.  Probably not an issue for the Secure world OS.  But for the Normal world OS, if it were compromised (and because of this risk you have TZ), the malicious code could easily prevent you ever switching back to the Secure world.

    The advantage to (1) is that the FIQ is beyond the control of Normal world OS, so if compromised, you can set enter the Secure world.
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  • Arm Employee Badge
    Note: This was originally posted on 11th October 2012 at http://forums.arm.com

    The basic problem with (2) is that it relies on both OSs (NWD and SWD) behaving.  Probably not an issue for the Secure world OS.  But for the Normal world OS, if it were compromised (and because of this risk you have TZ), the malicious code could easily prevent you ever switching back to the Secure world.

    The advantage to (1) is that the FIQ is beyond the control of Normal world OS, so if compromised, you can set enter the Secure world.
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