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

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi,

Answering to myself.

On 12/04/2017 02:30 PM, Julien Grall wrote:
> On 01/12/17 22:58, Stefano Stabellini wrote:
> > On Mon, 27 Nov 2017, Volodymyr Babchuk wrote:
> > = Xen command forwarding =
> >
> > In the code below, it looks like Xen is forwarding everything to OP-TEE.
> > Are there some commands Xen should avoid forwarding? Should we have a
> > whitelist or a blacklist?
> >
> >
> > = Long running OP-TEE commands and interruptions =
> >
> > I have been told that some OP-TEE RPC commands might take long to
> > complete. Is that right? Like for example one of the
> > OPTEE_MSG_RPC_CMD_*?
> >
> > If so, we need to think what to do in those cases. Specifically, do we
> > need a technique to restart certain commands in Xen, so that when they
> > run for too long and get interrupted by something (such as an
> > interrupt) we know how to restart them? In fact, do we need to setup a
> > timer interrupt to make sure the command doesn't block Xen for too long,
> > consuming the next vcpu's slot as well?
> I am not sure to understand your suggestion here. Where do you want that 
> timer? In Xen? If so, I don't think this could work. That's OP-TEE job 
> to break down long running operation.
> This is very similar to when a guest is doing an hypercall. Even if 
> setup a timer, the interrupt will only be received once the hypercall is 
> done (or Xen decided to preempt it).
So from Stuart's e-mail, I was slightly wrong about this. Most of the 
time a timer interrupt would get OP-TEE stopping his work and then 
return to the hypervisor. Although, this is still at the will of OP-TEE :).
The scheduler is already setting a timer interrupt to prevent a guest 
running outside of its slice. So I think this would do the job here to 
preempt OP-TEE.
Cheers,

--
Julien Grall

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