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

From: Julien Grall <julien.grall@linaro.org>
To: Stefano Stabellini <sstabellini@kernel.org>,
	Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Cc: xen-devel <xen-devel@lists.xenproject.org>, stuart.yoder@arm.com
Subject: Re: [RFC] WIP: optee: add OP-TEE mediator
Date: Wed, 6 Dec 2017 22:39:47 +0000	[thread overview]
Message-ID: <dc585624-e88b-389e-c34c-95778dd0ca9c@linaro.org> (raw)
In-Reply-To: <79401ae2-a697-948e-3a2b-1c618d32004c@linaro.org>

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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