Re: [Xen-devel] [RFC] WIP: optee: add OP-TEE mediator

[Stuart Yoder <stuart.yoder@xxxxxxx>]

On 12/6/17 4:31 PM, Julien Grall wrote:
> Hi Stuart,
>
> On 12/05/2017 03:36 PM, Stuart Yoder wrote:
> > > > There are limit on pCPUs, though. But this is not a problem, because
> > > > XEN scheduler will decide which guest will access OP-TEE right now.
> > > > OP-TEE don't have own scheduler at all, by the way. It is scheduled
> > > > by normal world.
> > > Do you mind to give a bit more explanation here? Do you plan to add knowledge 
> > > of OP-TEE in the scheduler?
> > Regarding scheduling-- OP-TEE runs with interrupts enabled (generally). So when 
> > an SMC
> > is in process in OP-TEE and the normal OS or hypervisor timer tick fires, 
> > OP-TEE halts
> > the current the in-progress thread, saves states, and returns to the normal 
> > world to
> > let the normal world timer interrupt handler and scheduler do its normal 
> > thing.  Eventually
> > when the normal world thread is re-scheduled and the in-progress thread 
> > restarts the
> > SMC.  That process continues until the SMC is completely done. So the OS/VMM 
> > scheduler
> > needs no awareness of OP-TEE since OP-TEE is cooperating with the normal world.
> Thank you for the explanation. I see you specifically mention the hypervisor 
> timer tick. How about the other interrupts? Will OP-TEE halts the current 
> in-progress thread and then return to Xen/OS?
Yes, that is my understanding.  There are a few "fast" SMC calls that run with 
interrupts
disabled, but other than that any other normal world interrupt will cause
OP-TEE to halt/exit and return to normal world.

Stuart

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