Re: xen/arm: Software Step ARMv8 - PC stuck on instruction

[James Morse <james.morse@arm.com>]

Hi Florian,

On 03/08/17 13:29, Florian Jakobsmeier wrote:
> So as far as I understood both of you don't see a general problem with
> (timer) interrupts or the scheduler while being single stepped? Because in
> my opinion after enabling singlestep the system will go into a "spinlock"
> routine.

Interrupts taken to EL2 will cause PSTATE.SS to be saved in SPSR_EL2.SS. This is
then restored by the ERET (provided Xen's PSTATE.D bit is set).

If its a virtual interrupt taken to EL1, you will end up stepping the interrupt
handler.


> Adapting your recommendations doesn't change the behavior.
> I'm still able to step over each instruction, but the control flow does not
> follow my module but rather executes my SMC to start SS and then enters the
> before mentioned procedure.

again?

SMC... Xen runs at EL2 so you must be trapping this. If the SMC is taken as trap
the ELR isn't updated to point to the instruction after the SMC, you have to do
this yourself. (See the 'note' for HCR_EL2.TSC in 'D1.15.3 EL2 configurable
controls')

SMC is also a corner case for single step. The PSTATE.SS bit isn't saved in the
SPSR. See Table D2-25 in 'D2.12.5 Behaviour in the active-not-pending state'.


Thanks,

James

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