Shared Memory in tz_vmm demo

[Joseph Lee]

Hi Martin,

Thank you so much.



On Tue, Apr 5, 2016 at 12:53 PM, Martin Stein <martin.stein at ...1...>
wrote:

> Hi Joseph,
>
> Am 05.04.2016 um 11:19 schrieb Joseph Lee:
> > Hi Stefan,
> >
> > Thanks a lot for your response.
> >
> > I have another questions. I have tried to measure the time for the
> > context switch between the worlds. I make an /SMC/ call in the normal
> > world (Linux) and modified the VMM to return to the normal world without
> > doing any operation. I take time t1 before calling the SMC instruction
> > and time t2 after the secure world switches back to the normal world.
> > Then the difference (t2 - t1) is the time for the context switch. i am
> > getting around /0.05/ milliseconds. Is that the right way to measure the
> > time for the context switch? FYI, i use /getrusage( )/ function in Linux
> > to measure t1 and t2.
> >
> > How do we measure time for a process in Genode. Is there a method to get
> > time in Genode?  Thanks again for you help and time.
>
> In general, your set-up looks reasonable to me. It is not granted that
> the scheduling order in your test case is exactly 'guest os -> vmm ->
> guest os'. You have to ensure that the secure world kernel doesn't mix
> in other subjects.
>

I just only commented out line 118 (i.e.,* if (_handle_vm())*) in
https://github.com/genodelabs/genode/blob/15.11/repos/os/src/server/tz_vmm/spec/imx53/main.cc
so that the TZ VMM switches to the normal world immediately by invoking the
run function of the VM session interface. I don't know how to do with
instruction re-ordering in secure world kernel. Can you please give me some
hint?


> Personally, I also would directly use a hardware timer instead of
> 'getrusage' to measure such small intervals. You could sample the timer
> via assembly instructions located directly before and after the SMC
> call, so no other instructions interfere. Make sure to prevent
> instruction re-ordering here also. The hardware as well as the compiler
> may under given circumstances re-order.
>

I wonder if you could tell me assembly instructions to measure small
intervals? i have tried the following but i got 0 value.


*      asm volatile ("MRC p15, 0, %0, c9, c13, 0\t\n": "=r"(t1));*

*        // smc call*
*      asm volatile ("MRC p15, 0, %0, c9, c13, 0\t\n": "=r"(t2));*

*      interval = t2 - t1;*



>
> You may already know, but if you like to do more detailed measurements,
> a timer that is configured non-secure allows you to sample from both
> worlds.
>
>    how do we measure the time taken to execute some instructions in the
secure world?


> Cheers,
> Martin
>


Thanks,



>
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