On 12.08.2016 08:43, Nikunj A Dadhania wrote: > David Gibson <da...@gibson.dropbear.id.au> writes: > >> [ Unknown signature status ] >> On Tue, Aug 09, 2016 at 02:47:46PM +0530, Nikunj A Dadhania wrote: >>> Nikunj A Dadhania <nik...@linux.vnet.ibm.com> writes: >>> >>>> David Gibson <da...@gibson.dropbear.id.au> 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