On (Mon) 31 Aug 2015 [20:46:02], Thomas Huth wrote:
> The PAPR interface provides a hypercall to pass high-quality
> hardware generated random numbers to guests. So let's provide
> this call in QEMU, too, so that guests that do not support
> virtio-rnd yet can get good random numbers, too.
virtio-rng, not rnd.
Can you elaborate what you mean by 'guests that do not support
virtio-rng yet'? The Linux kernel has had the virtio-rng driver since
2.6.26, so I'm assuming that's not the thing you're alluding to.
Not saying this hypercall isn't a good idea, just asking why. I think
there's are valid reasons like the driver fails to load, or the driver
is compiled out, or simply is loaded too late in the boot cycle.
> Please note that this hypercall should provide "good" random data
> instead of pseudo-random, so the function uses the RngBackend to
> retrieve the values instead of using a "simple" library function
> like rand() or g_random_int(). Since there are multiple RngBackends
> available, the user must select an appropriate backend via the
> "h-random" property of the the machine state to enable it, e.g.
>
> qemu-system-ppc64 -M pseries,h-random=rng-random ...
>
> to use the /dev/random backend, or "h-random=rng-egd" to use the
> Entropy Gathering Daemon instead.
I was going to suggest using -object here, but already I see you and
David have reached an agreement for that.
Out of curiosity: what does the host kernel use for its source when
going the hypercall route?
> +static void random_recv(void *dest, const void *src, size_t size)
> +{
> + HRandomData *hrcrdp = dest;
> +
> + if (src && size > 0) {
> + memcpy(&hrcrdp->val.v8[hrcrdp->received], src, size);
> + hrcrdp->received += size;
> + }
> + qemu_sem_post(&hrcrdp->sem);
> +}
> +
> +static target_ulong h_random(PowerPCCPU *cpu, sPAPRMachineState *spapr,
> + target_ulong opcode, target_ulong *args)
> +{
> + HRandomData hrcrd;
> +
> + if (!hrandom_rng) {
> + return H_HARDWARE;
> + }
> +
> + qemu_sem_init(&hrcrd.sem, 0);
> + hrcrd.val.v64 = 0;
> + hrcrd.received = 0;
> +
> + qemu_mutex_unlock_iothread();
> + while (hrcrd.received < 8) {
> + rng_backend_request_entropy((RngBackend *)hrandom_rng,
> + 8 - hrcrd.received, random_recv, &hrcrd);
> + qemu_sem_wait(&hrcrd.sem);
> + }
Is it possible for a second hypercall to arrive while the first is
waiting for the backend to provide data?
> + qemu_mutex_lock_iothread();
> +
> + qemu_sem_destroy(&hrcrd.sem);
> + args[0] = hrcrd.val.v64;
> +
> + return H_SUCCESS;
> +}
Amit
