On 02/09/15 09:48, David Gibson wrote: > On Wed, Sep 02, 2015 at 11:04:12AM +0530, Amit Shah wrote: >> 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.
Oh, sorry, I'll fix the description. >> 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. > > Yeah, I think we'd be talking about guests that just don't have it > configured, although I suppose it's possible someone out there is > using something earlier than 2.6.26 as well. Note that H_RANDOM has > been supported under PowerVM for a long time, and PowerVM doesn't have > any virtio support. So it is plausible that there are guests out > there with with H_RANDOM support but no virtio-rng support, although I > don't know of any examples specifically. RHEL6 had virtio support, > including virtio-rng more or less by accident (since it was only > supported under PowerVM). SLES may not have made the same fortunate > error - I don't have a system handy to check. Right, thanks David, I couldn't have explained it better. >>> 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? > > I believe it draws from the same entropy pool as /dev/random. The H_RANDOM handler in the kernel uses powernv_get_random_real_mode() in arch/powerpc/platforms/powernv/rng.c ... that seems to be a powernv-only pool (but it is also used to feed the normal kernel entropy pool, I think), but I am not an expert here so I might be wrong. >>> +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? > > Yes it is. The hypercall itself is synchronous, but you could get > concurrent calls from different guest CPUs. Hence the need for > iothread unlocking. BQL and semaphore handling should be ok, I think, but one remaining question is: Can the RngBackend deal with multiple requests in flight from different vCPUs? Or is it limited to one consumer only? Amit, do you know this? Thomas
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