Re: [Xen-devel] [PATCH v2 10/13] optee: add support for RPC commands

[Julien Grall <julien.grall@xxxxxxx>]

Hi Volodymyr,

On 09/11/2018 07:58 PM, Volodymyr Babchuk wrote:
> On 11.09.18 16:56, Julien Grall wrote:
> > On 10/09/18 19:14, Volodymyr Babchuk wrote:
> > > On 10.09.18 18:34, Julien Grall wrote:
> > > > On 03/09/18 17:54, Volodymyr Babchuk wrote:
> > > 1. OP-TEE issues RPC "allocate buffer"
> > > 2. NW returns list of pages
> > > 3. Mediator translates and stores address in non_contig[x]
> > > 4. OP-TEE begins to consume this list
> > > 5. IRQ arrives and OP-TEE forced to break the work
> > > 6. Mediator receives control back, but it should not free non_contig[x],
> > >     because it is not sure of OP-TEE finished reading from it
> > > 7. Xen/guest handles the IRQ and returns control back to OP-TEE
> > > 8. OP-TEE finishes processing this buffers and asks for another one
> > > 9. NW returns list of pages for the next buffer
> > > 10. At this point mediator is sure that OP-TEE finished processing
> > >      old non_contig[x], so it can free it and allocated another.
> > Thank you for the description of the protocol. However, it is still 
> > does not explain why you decided to free MAX_NONCONTIG_ENTRIES - 1. 
> > Why not 0 or 1 or n?
> Okay. You can pass up to 4 arguments for TA in command buffer. Any of that
> argument can be a shared memory reference, so we need at least 4 items
> in non_contig array. Sadly, this constant (TEE_NUM_PARAMS) is defined 
> not optee_msg.h or optee_smc.h, but in tee_api_defines.h, which is 
> user-space (or TA) part of OP-TEE.
> But, now I'm thinking, that maybe it is not so bad idea to add this file
> to XEN... What is your opinion?
I would just introduce TEE_NUM_PARAMS in tee.h with a comment explaining 
this is coming from GlobalPlatform TEE.
> Anyways, we need at least 4 items for arguments. But I defined 
> MAX_NONCONTIG_ENTRIES as 5, because last item in array is used for 
> RPC-allocated buffer. I also added comment. I'll copy it there:
> /* Last entry in non_contig array is used to hold RPC-allocated  buffer */
>
> So, first 4 items are used for arguments and last one used for RPC 
> requests. Right way to define MAX_NONCONTIG_ENTRIES is (TEE_NUM_PARAMS + 
> 1), but then I should add another header file, which defines 
> GlobalPlatform TEE core API.
If I understand correctly your e-mail, a TEE call can never have more 
than 4 parameters. Right? If so, should not your code mostly use 
TEE_NUM_PARAMS and not MAX_NONCONTIG_ENTRIES?
> > Overall, it feels like to me you want to write more documentation 
> > about how the mediator is supposed to work.
> Looks like I need to start on how OP-TEE protocol is supposed to work...
> I tried to cover this in the commit messages, but looks like it is not 
> sufficient.
I appreciate your effort to cover that in the commit message :). I tend 
to update commit message over revision when there are misunderstanding 
on the patch.
Cheers,

--
Julien Grall

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