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Invalid opcode exception error for HVC instruction

Nicola Johnsen
Note: This was originally posted on 9th November 2012 at http://forums.arm.com

Hi I'm developing a UEFI boot loader that can run a hypervisor. I've developed some code that initializes CPUs and then transitions from monitor mode into non-secure hyp mode. I've set up HVBAR etc but when I then call the hvc #0 assembly instruction, the runtime (UEFI tianocore on ARM Fast model simulator) reports an Invalid Opcode exception. I wonder what could be the reason for this exception? I'm using a Cortex-A15 so the instruction should be valid. I've also set SCE.HCE to 1 so the hvc instruction should be available in non-secure monitor mode. Is there anything else I'm missing here? I've already asked for help from the UEFI people but didn't get any response so I'm trying a different audience.

Cheers.
  • Note: This was originally posted on 9th November 2012 at http://forums.arm.com

    Hi there, thanks for your reply. I think I have actually fixed the problem. Initially I was in monitor mode with SCR.NS bit set to 1 (so non-secure state). But that didn't work. Then I switched to SVC with NS bit still set to 1 and it worked. From reading the ARMv7 specs, that isn't quite what I was expecting. In the specs it says that you can use HVC in monitor mode as well in order to switch to HYP mode, as long as the NS bit is set to 1. Is that not correct?
  • Arm Employee Badge
    Note: This was originally posted on 9th November 2012 at http://forums.arm.com

    What mode is the processor in at the point you execute the HVC? And what is the value of SCR.NS bit?
  • Arm Employee Badge
    Note: This was originally posted on 12th November 2012 at http://forums.arm.com

    Actually what you described is what I had expected.  If you look at section B9.3.4 of the architecture reference manual (  rev C.b  ) it gives the pseudo code for the HVC instruction.  First bit reads:

    if !HasVirtExt() || IsSecure() || !CurrentModeIsNotUser() then UNDEFINED;

    Mon mode is always classed as secure, so you were falling over the the IsSecure() test.