linux-kernel - Re: [PATCH 1/2] tee: system invocation

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Date:   Thu, 9 Feb 2023 10:11:56 +0100
From:   Etienne Carriere <etienne.carriere@...aro.org>
To:     Jens Wiklander <jens.wiklander@...aro.org>
Cc:     Sumit Garg <sumit.garg@...aro.org>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Sudeep Holla <sudeep.holla@....com>,
        Cristian Marussi <cristian.marussi@....com>
Subject: Re: [PATCH 1/2] tee: system invocation

On Thu, 9 Feb 2023 at 09:11, Etienne Carriere
<etienne.carriere@...aro.org> wrote:
>
> Hi Jens,
>
>
> On Thu, 9 Feb 2023 at 08:14, Jens Wiklander <jens.wiklander@...aro.org> wrote:
> >
> > Hi Etienne,
> >
> > On Wed, Feb 08, 2023 at 06:09:17PM +0100, Etienne Carriere wrote:
> > > Hello Sumit, Jens,
> > >
> > [snip]
> > > > > > > >
> > > > > > > >         if  (rpc_arg && tee_shm_is_dynamic(shm)) {
> > > > > > > > -               param.a0 = OPTEE_SMC_CALL_WITH_REGD_ARG;
> > > > > > > > +               if (ctx->sys_service &&
> > > > > > > > +                   (optee->smc.sec_caps & OPTEE_SMC_SEC_CAP_SYSTEM_THREAD))
> > > > > > > > +                       param.a0 = OPTEE_SMC_CALL_SYSTEM_WITH_REGD_ARG;
> > > > > > > > +               else
> > > > > > > > +                       param.a0 = OPTEE_SMC_CALL_WITH_REGD_ARG;
> > > > > > >
> > > > > > > This system thread flag should also be applicable to platforms without
> > > > > > > registered arguments support. IOW, we need similar equivalents for
> > > > > > > OPTEE_SMC_FUNCID_CALL_WITH_ARG and OPTEE_SMC_FUNCID_CALL_WITH_RPC_ARG
> > > > > > > too. So I would rather suggest that we add following flag to all 3
> > > > > > > call types:
> > > > > > >
> > > > > > > #define OPTEE_SMC_CALL_SYSTEM_THREAD_FLAG    0x8000
> > > > > >
> > > > > > The main reason platforms don't support registered arguments is that
> > > > > > they haven't been updated since this was introduced. So if a platform
> > > > > > needs system threads it could update to use registered arguments too.
> > > > >
> > > > > Are we hinting at deprecating reserved shared memory support? If yes,
> > > > > wouldn't it be better to be explicit about it with a boot time warning
> > > > > message about its deprecation?
> > > > >
> > > > > Otherwise it will be difficult to debug for the end user to find out
> > > > > why system thread support isn't activated.
> > > > >
> > > > > > The Linux kernel already supports registered arguments. An advantage
> > > > > > with the current approach is that the ABI is easier to implement
> > > > > > since we have distinct SMC IDs for each function.
> > > > >
> > > > > I see your point but my initial thought was that we don't end up
> > > > > making that list too large that it becomes cumbersome to maintain,
> > > > > involving all the combinatorial.
> > > >
> > > > You have a point. Etienne, do you think we could give it a try at
> > > > https://github.com/OP-TEE/optee_os/pull/5789 to better see how this
> > > > would play out?
> > > >
> > >
> > > Indeed I miss that...
> > > With the patch proposed here, indeed if OP-TEE does not support
> > > dynamic shared memory then Linux will never use the provisioned TEE
> > > thread. This is weird as in such a case OP-TEE provisions resources
> > > that will never be used, which is the exact opposite goal of this
> > > feature. Verified on our qemu-arm setup.
> > >
> > > For simplicity, I think this system invocation should require OP-TEE
> > > supports dyn shm.
> >
> > It's not obvious to me that this will easier to implement and maintain.
> > Looking at the code in optee_os it looks like using a flag bit as
> > proposed by Sumit would be quite easy to handle.
>
> OP-TEE could auto disable thread provis when dyn shm is disabled, right.

By the way, from OP-TEE OS, we have config switches for dyn-shm and
system context provisioning.
The latter could depend on the former so it's clear at build time when
TEE can embed the capability.

Br,
etienne

> Will it be sufficient? We will still face cases where an OP-TEE
> provisions thread but Linux kernel is not aware (older vanilla kernel
> used with a recent OP-TEE OS). Not much platforms are really affected
> I guess but those executing with pager in small RAMs where a 4kB
> thread context costs.
>
> (snip)