synchronization between normal and secure world

[Stefan Brenner]

On 05.05.2015 11:29, Stefan Kalkowski wrote:
> Hi,
> 
> On 04/28/2015 02:26 PM, Stefan Brenner wrote:
>> Hi,
>>
>> just a short question, I am using the TZ VMM example on i.MX53. How can
>> I syncronize accesses to a shared memory range between secure world and
>> normal world? I would be interested in a mutex that can be acquired both
>> from Genode in secure world, as well as from Linux in normal world.
> 
>>From the secure world's side, the synchronization is somehow implicitly
> available. Whenever the VMM receives a VM exception signal, it knows
> that the VM is in pause state and cannot intercept it. Moreover, if the
> VMM receives other asynchronous events (e.g. events from some backend
> devices etc.) it can call pause() on the VM object. If the pause() call
> returns, the VMM can be sure that the VM is not scheduled anymore. By
> calling run() after a critical path the VMM makes the VM executable again.
> 
>>From the normal world's perspective you might ask the secure world via a
> hypercall explicitly when entering or leaving a critical section.
> 
> Of course, you can also implement an inter-world capable mutex laying
> within a shared memory region that is mapped uncached in both worlds. By
> now we did not need such a mutex for our experimental VMM. Therefore,
> there is no implementation existent.

Thanks a lot for your answer! This sounds good for me, at least from the
secure world's perspective. However, the other way round, it would be
way too much overhead for my normal world application to switch back and
forth to secure world twice (once for lock and once for unlock) for each
critical section.

It would be more suitable for my app to not do the critical action in
normal world, but ask the secure world to do it. Then it would be
synchronized as well, but only one world switch back and forth necessary.

I think I have to bite the apple and implement what you mention, a mutex
in a shared memory section. Do you have any further input for me
according to this? Especially about the "mapped uncached" property?

> 
> Regards
> Stefan
> 
>>
>> Ciao
>>
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