Genode, TrustZone and Android on i.mx6

[Vincent Raes]

Hello,

I am currently working on a TrustZone demo using Genode in the Secure world
and an Android OS in the Normal world on the SABRElite board.


I've succesfully got the Genode 15.08 base-hw kernel working on the board
but I am running in some issues with the Normal world. For this I am using
an Android distribution customised for the SABRElite, provided by the SoC
distributor.


One major problem is that I'm unable to get any output from the Android OS
when it is booted by Genode. When booting Android using u-boot, there is
output as expected. This supports the idea that the used Device Tree Blob
is correct. The command line options for the kernel are provided during its
compilation. This seems to work since booting with u-boot gets output on
the specified ports.


When booting with Genode however, there is no output whatsoever. The
location of the DTB in the memory is passed to the Normal world using the
r2 register, which as far as I know is all that needs to be done.

Any input on this would be much appreciated since you can probably imagine
that debugging the Normal world kernel without output is a grueling task


For now there exists a workaround for debugging using kernel output, thanks
to the Secure world. The current debug procedure consists of entering the *smc
*command in the code to check whether the Normal world reaches this portion
of code. This has allowed me to get the kernel itself running but here I
ran into another issue, being the L2 cache used in the SABRElite board.

The Linux kernel tries to configure which can't really be done from the
Normal world. It can however make calls to the Secure world to configure
the restricted registers. Once the cache is actually enabled, Genode
immediately stops though. I imagine this is caused by the fact that Genode
does not expect the L2 cache to actually work.

Are there any pointers to correctly configure the L2 cache in Genode to
keep it executing after the cache's activation? Alternatively, is it
possible to run Android without the L2 cache and how would one best go
about achieving this?

Best regards,

Vincent

[Stefan Kalkowski]

Hi Vincent,

Post by Vincent Raes
Hello,
I am currently working on a TrustZone demo using Genode in the Secure world
and an Android OS in the Normal world on the SABRElite board.
I've succesfully got the Genode 15.08 base-hw kernel working on the board
but I am running in some issues with the Normal world. For this I am using
an Android distribution customised for the SABRElite, provided by the SoC
distributor.
One major problem is that I'm unable to get any output from the Android OS
when it is booted by Genode. When booting Android using u-boot, there is
output as expected. This supports the idea that the used Device Tree Blob
is correct. The command line options for the kernel are provided during its
compilation. This seems to work since booting with u-boot gets output on
the specified ports.
When booting with Genode however, there is no output whatsoever. The
location of the DTB in the memory is passed to the Normal world using the
r2 register, which as far as I know is all that needs to be done.
Any input on this would be much appreciated since you can probably imagine
that debugging the Normal world kernel without output is a grueling task

Yes, I know what you mean. I'm afraid this is the fortune of a system
developer ;-). You can however check, whether you've enabled "early
printk" in your Linux kernel configuration and add "earlyprintk" to your
kernel command line.
Nevertheless, if your guest kernel stucks before the first outputs
during CPU initialization you might follow the way you already used when
incorporating 'smc' calls into the kernel code. I used the same
mechanism when doing first steps with TrustZone and "guest kernels"
running in the normal world.

Post by Vincent Raes
For now there exists a workaround for debugging using kernel output, thanks
to the Secure world. The current debug procedure consists of entering the *smc
*command in the code to check whether the Normal world reaches this portion
of code. This has allowed me to get the kernel itself running but here I
ran into another issue, being the L2 cache used in the SABRElite board.
The Linux kernel tries to configure which can't really be done from the
Normal world. It can however make calls to the Secure world to configure
the restricted registers. Once the cache is actually enabled, Genode
immediately stops though. I imagine this is caused by the fact that Genode
does not expect the L2 cache to actually work.

Yeah, I also realized recently that the i.MX6 specific code, which was
contributed to Genode, does not make use of the L2 cache at all. In
contrast to the Freescale boards, on all other Cortex A9 centered ARM
platforms the external L2 cache was already enabled by firmware or
u-boot, therefore Genode's base-hw kernel did not take care.
When realizing that, I also tried to enable the L2 cache on i.MX6 using
the Wandboard. Unfortunately it is much more complex because they used a
rather old revision of Cortex A9 for their SoC with a complex
initialization routine regarding SMP and cache coherency hardware. I
finished the work yesterday:

https://github.com/skalk/genode/tree/issue%231312

Nevertheless, I first have to cleanup the topic branch before it will
hit the staging branch of Genode's mainline repository within the next days.

Post by Vincent Raes
Are there any pointers to correctly configure the L2 cache in Genode to
keep it executing after the cache's activation? Alternatively, is it
possible to run Android without the L2 cache and how would one best go
about achieving this?

You can simply deactivate the L2 cache initialization within the Linux
code, and continue your development. Of course, memory access is much
much slower without the L2 cache (~ factor 10 here) and Android won't be
much useable. Nevertheless, as soon as the L2 cache enabling for i.MX6
enters the staging branch you might switch to that state..

Regards
Stefan

Post by Vincent Raes
Best regards,
Vincent
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