[Gnuk-users] Answer to Will you support gnuk on STM32F4Discovery?

[NIIBE Yutaka]

On 11/08/2016 01:39 AM, Bastien ROUCARIES wrote:
> I plan to port to olimex stm32f4 board and I plan to volontuer.
> 
> Could you help me to port ?
> 
> I think I should begin with Chopstx ?
> 
> Could you give me some pointer ?

For myself, I don't have strong interest porting Gnuk to a chip with
Cortex-M4 like STM32F4.  It's a larger chip with more complexity,
which means that the possibility of attacks will be increased.  (My
interest is porting Gnuk to Cortex-M0plus.  That is: smaller chip with
less complexity, importantly, which comes with a fixed number of
cycles for MUL operation.)

IIRC, Kaz Kojima did some work for porting Chopstx to a chip with
Cortex-M4 and another chip of Cortex-A9.  For the latter (multi
cores), we once tried to merge, but the merge had not finished.  Only
some parts were merged.  Meanwhile Chopstx introduced new API.  We
need to resume porting work again (for multiple cores, or more portable
Chopstx).

For Cortex-M4, the handling of FPU is the major issue.  I suggested
Kaz that FPU registers are handled by a user application (if it's
possible), not by Chopstx itself.  If use of FPU can be done by only
a thread (like a device driver), it will be easier to manage.  If not,
more stack space will be needed to save possible FPU registers' store.
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