Re: [Qemu-devel] [PATCH v3] ppc/spapr: Implement H_RANDOM hypercall in QEMU

[Thomas Huth]

On 14/09/15 04:15, David Gibson wrote:
> On Fri, Sep 11, 2015 at 11:17:01AM +0200, Thomas Huth wrote:
>> The PAPR interface defines a hypercall to pass high-quality
>> hardware generated random numbers to guests. Recent kernels can
>> already provide this hypercall to the guest if the right hardware
>> random number generator is available. But in case the user wants
>> to use another source like EGD, or QEMU is running with an older
>> kernel, we should also have this call in QEMU, so that guests that
>> do not support virtio-rng yet can get good random numbers, too.
>>
>> This patch now adds a new pseude-device to QEMU that either
>> directly provides this hypercall to the guest or is able to
>> enable the in-kernel hypercall if available. The in-kernel
>> hypercall can be enabled with the use-kvm property, e.g.:
>>
>>  qemu-system-ppc64 -device spapr-rng,use-kvm=true
>>
>> For handling the hypercall in QEMU instead, a RngBackend is required
>> since the hypercall should provide "good" random data instead of
>> pseudo-random (like from 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 "backend"
>> property of the device, e.g.:
>>
>>  qemu-system-ppc64 -object rng-random,filename=/dev/hwrng,id=rng0 \
>>                    -device spapr-rng,backend=rng0 ...
>>
>> See http://wiki.qemu-project.org/Features-Done/VirtIORNG for
>> other example of specifying RngBackends.
...
>> +
>> +#include "qemu/error-report.h"
>> +#include "sysemu/sysemu.h"
>> +#include "sysemu/device_tree.h"
>> +#include "sysemu/rng.h"
>> +#include "hw/ppc/spapr.h"
>> +#include "kvm_ppc.h"
>> +
>> +#define SPAPR_RNG(obj) \
>> +    OBJECT_CHECK(sPAPRRngState, (obj), TYPE_SPAPR_RNG)
>> +
>> +typedef struct sPAPRRngState {
>> +    /*< private >*/
>> +    DeviceState ds;
>> +    RngBackend *backend;
>> +    bool use_kvm;
>> +} sPAPRRngState;
>> +
>> +typedef struct HRandomData {
>> +    QemuSemaphore sem;
>> +    union {
>> +        uint64_t v64;
>> +        uint8_t v8[8];
>> +    } val;
>> +    int received;
>> +} HRandomData;
>> +
>> +/* Callback function for the RngBackend */
>> +static void random_recv(void *dest, const void *src, size_t size)
>> +{
>> +    HRandomData *hrdp = dest;
>> +
>> +    if (src && size > 0) {
>> +        assert(size + hrdp->received <= sizeof(hrdp->val.v8));
>> +        memcpy(&hrdp->val.v8[hrdp->received], src, size);
>> +        hrdp->received += size;
>> +    }
>> +
>> +    qemu_sem_post(&hrdp->sem);
> 
> I'm assuming qemu_sem_post() includes the necessary memory barrier to
> make sure the requesting thread actually sees the data.

Not sure whether I fully got your point here... both callback function
and main thread are calling an extern C-function, so the compiler should
not assume that the memory stays the same in the main thread...?

Anyway, I've tested the hypercall by implementing it in SLOF and calling
it a couple of times there to see that all bits in the result behave
randomly, so for me this is working fine.

>> +}
>> +
>> +/* Handler for the H_RANDOM hypercall */
>> +static target_ulong h_random(PowerPCCPU *cpu, sPAPRMachineState *spapr,
>> +                             target_ulong opcode, target_ulong *args)
>> +{
>> +    sPAPRRngState *rngstate;
>> +    HRandomData hrdata;
>> +
>> +    rngstate = SPAPR_RNG(object_resolve_path_type("", TYPE_SPAPR_RNG, 
>> NULL));
>> +
>> +    if (!rngstate || !rngstate->backend) {
>> +        return H_HARDWARE;
>> +    }
>> +
>> +    qemu_sem_init(&hrdata.sem, 0);
>> +    hrdata.val.v64 = 0;
>> +    hrdata.received = 0;
>> +
>> +    qemu_mutex_unlock_iothread();
>> +    while (hrdata.received < 8) {
>> +        rng_backend_request_entropy(rngstate->backend, 8 - hrdata.received,
>> +                                    random_recv, &hrdata);
>> +        qemu_sem_wait(&hrdata.sem);
>> +    }
>> +    qemu_mutex_lock_iothread();
>> +
>> +    qemu_sem_destroy(&hrdata.sem);
>> +    args[0] = hrdata.val.v64;
>> +
>> +    return H_SUCCESS;
>> +}
>> +
>> +static void spapr_rng_instance_init(Object *obj)
>> +{
>> +    sPAPRRngState *rngstate = SPAPR_RNG(obj);
>> +
>> +    if (object_resolve_path_type("", TYPE_SPAPR_RNG, NULL) != NULL) {
>> +        error_report("spapr-rng can not be instantiated twice!");
>> +        return;
>> +    }
>> +
>> +    object_property_add_link(obj, "backend", TYPE_RNG_BACKEND,
>> +                             (Object **)&rngstate->backend,
>> +                             object_property_allow_set_link,
>> +                             OBJ_PROP_LINK_UNREF_ON_RELEASE, NULL);
>> +    object_property_set_description(obj, "backend",
>> +                                    "ID of the random number generator 
>> backend",
>> +                                    NULL);
> 
> Since virtio-rng does it the same way, I'm assuming there's a reason
> this is constructed with object_propery_add() rather than listing it
> in spapr_rng_properties, but it's not obvious what the reason is.

I did not spot a macro a la "DEFINE_PROP_LINK" that could be used for
this. Do you see a possibility to define a link that way?

> More importantly, this should probably be called "rng" not "backend"
> to match virtio-rng.

Since the device is already called "spapr-rng", i.e. has "rng" in its
name, I'd rather like to keep this as "backend" to make it clear that
you specify the backend this way.

>> +}
>> +
>> +static void spapr_rng_realize(DeviceState *dev, Error **errp)
>> +{
>> +
>> +    sPAPRRngState *rngstate = SPAPR_RNG(dev);
>> +
>> +    if (rngstate->use_kvm) {
>> +        if (kvmppc_enable_hwrng() != 0) {
>> +            error_setg(errp, "Could not initialize in-kernel H_RANDOM 
>> call!");
>> +        }
>> +        return;
>> +    }
>> +
>> +    if (!rngstate->backend) {
>> +        error_setg(errp, "spapr-rng needs a RNG backend!");
>> +        return;
>> +    }
> 
> So, the logic here means you have to explicitly choose whether to use
> the kernel implementation or the qemu imeplementation.
> 
> It seems to me it might be useful to be able to specify "use the
> kernel implementation if available, otherwise fall back to qemu".

Right, makes sense, I'll update this logic.

Thanks for the review,
 Thomas

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