Re: [Qemu-devel] [PATCH] aspeed_scu: Implement RNG register

["Cédric Le Goater" <clg@kaod.org>]

On 05/28/2018 04:29 PM, Joel Stanley wrote:
> On 28 May 2018 at 23:33, Joel Stanley <joel@jms.id.au> wrote:
>> On 28 May 2018 at 23:17, Cédric Le Goater <clg@kaod.org> wrote:
>>> Hello Joel,
>>>
>>> On 05/28/2018 02:46 PM, Joel Stanley wrote:
>>>> The ASPEED SoCs contain a single register that returns random data when
>>>> read. This models that register so that guests can use it.
>>>>
>>>> Signed-off-by: Joel Stanley <joel@jms.id.au>
>>>> ---
>>>>  hw/misc/aspeed_scu.c | 19 +++++++++++++++++++
>>>>  1 file changed, 19 insertions(+)
>>>>
>>>> diff --git a/hw/misc/aspeed_scu.c b/hw/misc/aspeed_scu.c
>>>> index 5e6d5744eeca..8fa0cecf0fa1 100644
>>>> --- a/hw/misc/aspeed_scu.c
>>>> +++ b/hw/misc/aspeed_scu.c
>>>> @@ -16,6 +16,7 @@
>>>>  #include "qapi/visitor.h"
>>>>  #include "qemu/bitops.h"
>>>>  #include "qemu/log.h"
>>>> +#include "crypto/random.h"
>>>>  #include "trace.h"
>>>>
>>>>  #define TO_REG(offset) ((offset) >> 2)
>>>> @@ -154,6 +155,18 @@ static const uint32_t ast2500_a1_resets[ASPEED_SCU_NR_REGS] = {
>>>>       [BMC_DEV_ID]      = 0x00002402U
>>>>  };
>>>>
>>>> +static uint32_t aspeed_scu_get_random(void)
>>>> +{
>>>> +    Error *err = NULL;
>>>> +    uint32_t num;
>>>> +
>>>> +    if (qcrypto_random_bytes((uint8_t *)&num, sizeof(num), &err)) {
>>>> +        error_report_err(err);
>>>> +    }
>>>> +
>>>> +    return num;
>>>> +}
>>>> +
>>>>  static uint64_t aspeed_scu_read(void *opaque, hwaddr offset, unsigned size)
>>>>  {
>>>>      AspeedSCUState *s = ASPEED_SCU(opaque);
>>>> @@ -167,6 +180,9 @@ static uint64_t aspeed_scu_read(void *opaque, hwaddr offset, unsigned size)
>>>>      }
>>>>
>>>>      switch (reg) {
>>>> +    case RNG_DATA:
>>>> +        return aspeed_scu_get_random();
>>>
>>> may be we could test bit 1 of RNG_CTRL to check if it is enabled or not.
>>
>> The RNG is enabled by default, and I didn't find any software that
>> disables it, but it can't hurt to have that check.
> 
> I did some testing on hardware, and the rng still outputs a different
> number each time I ask for one regardless of the state of the enabled
> bit.
> 
> How should we model that?
> 

I confirm that the HW doesn't really care about the enabled bit.
Let's ignore it then ?

C.