Messages in this thread | | | From | Etienne Carriere <> | Date | Thu, 9 Feb 2023 13:56:54 +0100 | Subject | Re: [PATCH 1/2] tee: system invocation |
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On Thu, 9 Feb 2023 at 10:19, Jens Wiklander <jens.wiklander@linaro.org> wrote: > > Hi, > > On Thu, Feb 09, 2023 at 09:11:53AM +0100, Etienne Carriere wrote: > > Hi Jens, > > > > > > On Thu, 9 Feb 2023 at 08:14, Jens Wiklander <jens.wiklander@linaro.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. > > 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. > > When you add exceptions you make it more complicated. Now we must > remember to always use dyn shm if we are to succeed in configuring with > system threads. What if both dyn shm and static shm is configured in > OP-TEE, but the kernel only uses static shm? > > > > > If OP-TEE could know when Linux does not support TEE system > > > > invocation, then OP-TEE could let any invocation use these provisioned > > > > resources so that they are not wasted. > > > > I think a good way would be Linux to expose if it supports this > > > > capability, during capabilities exchange. > > > > Would you agree with this approach? > > > > > > No, I'm not so keen on adding that side effect to > > > OPTEE_SMC_EXCHANGE_CAPABILITIES. > > > > It is a capability REE would exchanges with TEE. > > What kind of side effects do you fear? > > I was hoping to keep it stateless. One thing less to keep track of when > handing over from a boot stage to the kernel.
Or from a kernel VM unload/reload.
> > > > The way you're describing the problem it sounds like it's a normal world > > > problem to know how many system threads are needed. How about adding a > > > fast call where normal world can request how many system threads should > > > be reserved? If none are requested, none will be reserved. > > > > Well, could be. With caps exchange, we have an SMC funcID to REE to > > say to TEE: "reserved the default configured number of sys thread". I > > think it is simpler. > > Until you realize the that the default number of system threads doesn't > match what you need.
Ok, I see your point. Indeed, Linux drivers requiring system context could issue a fastcall SMC to request dynamic provisioning of TEE context resources, and release their request upon driver unload. I agree it would better scale in the long term. I'll propose something in a v2.
> > > > > With REE calling TEE to provision thread, we would need another call > > to release the reservation. Whe caps exchange, we have a single SMC to > > reconfig the negotiated caps. > > A single SMC with growing complexity in its arguments.
:) fair.
> > Cheers, > Jens
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