Re: [PATCH v2 09/13] optee: add support for arbitrary shared memory

[Julien Grall <julien.grall@arm.com>]

On 09/11/2018 08:33 PM, Volodymyr Babchuk wrote:
> Hi Julien,

Hi,

> On 11.09.18 16:37, Julien Grall wrote:
>> Hi Volodymyr,
>>
>> On 10/09/18 19:04, Volodymyr Babchuk wrote:
>>> On 10.09.18 17:02, Julien Grall wrote:
>>>> On 03/09/18 17:54, Volodymyr Babchuk wrote:
> [...]
>>>>
>>>>> +    if ( !pages_data_xen_start )
>>>>> +        return false;
>>>>> +
>>>>> +    shm_buf = allocate_shm_buf(ctx, param->u.tmem.shm_ref, 
>>>>> num_pages);
>>>>
>>>> In alocate_shm_buf you are now globally limiting the number of pages 
>>>> ( (16384) to pin. However, this does not limit per call.
>>>>
>>>> With the current limit, you would could call up to 16384 times 
>>>> lookup_and_pin_guest_ram_addr(...). On Arm, for p2m related 
>>>> operation, we limit to 512 iterations in one go before checking the 
>>>> preemption.
>>>> So I think 16384 times is far too much.
>>> So, in other words, I can translate only 2MB buffer (if 4096KB pages 
>>> are used), is it right?
>>
>> 2MB for the whole command. So if you have 5 buffer in the command, 
>> then the sum of the buffer should not be bigger than 2MB.
> 4  buffers, but yes, it can be up to 8MB. Okay, I'll add per-call 
> counter to limit memory usage for a whole call.

That would need to be reduced to 2MB in total per call. You probably 
want to look at max_order(...).

> 
>> However, 2MB might be too big considering that you also need to 
>> account the SMC call. Does buffer can be passed for fast call?
> No, all such calls are yielding calls, so you can ignore time used for 
> SMC call itself.

How come you can ignore it? It has a cost to trap to EL3.

Cheers,

-- 
Julien Grall

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