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

[Nikunj A Dadhania]

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.

Regards
Nikunj