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

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

Hi Florian, Julien,

On 02/08/17 14:32, Julien Grall wrote:
> On 26/07/17 14:12, Florian Jakobsmeier wrote:
>> i was just testing the single step implementation and realized that the
>> before mentioned solution is not fully working. I'm still trying to
>> enable SS for a VM on Xen.

>> To test my implementation i wrote a small Kernel Module and started it
>> in the DomU. The module only contains a loop which increments a counter
>> and prints its value.
>> Right after loading the moduleI start the single step mechanism in the
>> Dom0 for the VM (again with xen-access).
>> As soon as i start the SS the VM will stop working.

>> The ARM ARM (D2-1956 - ARM DDI 0487B.a ID033117) states that, in order
>> to enables software step:
>>
>>     A debugger enables MDSCR_EL1.SS = 1
>>
>>     Executes an ERET

...with SPSR.SS = 1, and you need to ERET with PSTATE.D disabled (which I assume
Xen always has).

This then becomes the guest's PSTATE.SS bit, which suppresses the single-step
exception until it has stepped one instruction.


>>     The PE executes the instruction to be single-stepped
>>
>>     Takes a software step exception on the next instruction


>> My guess is that by setting the needed SS registers ever time when we
>> leave the guest, the configuration won't allow the guest to execute the
>> "to be single stepped instruction"
>> Before executing the (first) instruction the VM will generate the SS
>> exception (as desired). In the hypervisor we will set the SS registers
>> again, which could hinder the VM to execute the instruction (which we
>> want because we already generated an SS exception for this instruction)
>> and instead generate a second SS exception for it. This will lead to the
>> second PC print in the single step handler

>> But I'm not able to find any proof for this.

I'm afraid the ARM-ARM could be clearer about how this works. (It's had me
confused all week!).

The state machine in figure D2-4 (D2.12.3 the software step state machine)
should help. I haven't looked at Xen (or your patch), but from your description
it sounds like you are entering the guest with the debug state machine in
active-pending state, then taking a single-step exception immediately. You never
actually execute the instruction-to-be-stepped.

Instead you want to enter the guest in active-not-pending state, the rules for
this are in 'D2.12.4 Entering the active-not-pending state'.


Thanks,

James

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