Re: [Qemu-devel] [Qemu-ppc] [PATCH 2/6] target-ppc: Implement darn instruction

[Thomas Huth]

On 12.08.2016 08:43, Nikunj A Dadhania wrote:
> David Gibson <address@hidden> writes:
> 
>> [ Unknown signature status ]
>> On Tue, Aug 09, 2016 at 02:47:46PM +0530, Nikunj A Dadhania wrote:
>>> Nikunj A Dadhania <address@hidden> writes:
>>>
>>>> David Gibson <address@hidden> writes:
>>>>
>>>>> [ Unknown signature status ]
>>>>> On Mon, Aug 08, 2016 at 07:33:37AM +1000, Benjamin Herrenschmidt wrote:
>>>>>> On Sun, 2016-08-07 at 23:06 +0530, Nikunj A Dadhania wrote:
>>>>>>> +target_ulong helper_darn(uint32_t l)
>>>>>>> +{
>>>>>>> +    target_ulong r = UINT64_MAX;
>>>>>>> +
>>>>>>> +    if (l <= 2) {
>>>>>>> +        do {
>>>>>>> +            r = random() * random();
>>>>>>> +            r &= l ? UINT64_MAX : UINT32_MAX;
>>>>>>> +        } while (r == UINT64_MAX);
>>>>>>> +    }
>>>>>>> +
>>>>>>> +    return r;
>>>>>>> +}
>>>>>>>  #endif
>>>>>>
>>>>>> Isn't this a bit week ? Look at the implementation of H_RANDOM...
>>>>>
>>>>> Indeed, you should be using the rng backend that H_RANDOM, virtio-rng
>>>>> and the Intel random number instruction all use.
>>>
>>> Can you point me to the intel instruction, I couldn't get rdrand
>>> implementation.
>>
>> Ah.. turns out no.  I'd assumed it was there and used the same backend
>> as virtio-rng and H_RANDOM, but I hadn't actually looked at the code,
>> and now that I'm trying I can't see it either.
>>
>>>
>>>> I was looking at implementing this, AFAIU, I have to get a new RNG
>>>> object in the initialization routine. We would need an instance of this
>>>> per machine. So for pseries I can add in ppc_spapr_init(). I am not sure
>>>> in case of linux-user where should this be initialized.
>>>>
>>>> One other place was init_proc_POWER9(), but that will be per cpu and
>>>> member of CPUPPCState structure. Advantage is it will work for system
>>>> emulation and linux-user both and we would not need a lock.
>>>
>>> More issues here. Random backend is not compiled for linux-user, adding
>>> that wasn't difficult, but then rng_backend_request_entropy() uses
>>> qemu_set_fd_handler() which is a stub for linux-user
>>> (stubs/set-fd-handler.c)7
>>
>>
>> Ah.. yeah, not sure how we'll need to handle that.
> 
> I have sent updated patch, reading from /dev/random. Not sure if that is
> allowed in tcg. Works fine though.

You can not rely on /dev/random for this job, since it might block. So
your guest would stop executing when there is not enough random data
available in the host, and I think that's quite a bad behavior...

 Thomas