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

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?


Julien Grall

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