[RFC] rng-builtin: add an RNG backend that uses qemu_guest_getrandom()

Add a new RNG backend using QEMU builtin getrandom function.

It can be created with "-object rng-builtin".

This patch applies on top of
"[PATCH v4 00/24] Add qemu_getrandom and ARMv8.5-RNG etc"
Based-on: <20190506173353.32206-1-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 backends/Makefile.objs |  2 +-
 backends/rng-builtin.c | 56 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 57 insertions(+), 1 deletion(-)
 create mode 100644 backends/rng-builtin.c

On Thu, May 09, 2019 at 04:23:42PM +0200, Laurent Vivier wrote:
> Add a new RNG backend using QEMU builtin getrandom function.
> 
> It can be created with "-object rng-builtin".
> 
> This patch applies on top of
> "[PATCH v4 00/24] Add qemu_getrandom and ARMv8.5-RNG etc"
> Based-on: <20190506173353.32206-1-richard.henderson@linaro.org>

These 3 lines ought to be below the "---" since they're not
relevant once merged.

> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  backends/Makefile.objs |  2 +-
>  backends/rng-builtin.c | 56 ++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 57 insertions(+), 1 deletion(-)
>  create mode 100644 backends/rng-builtin.c

qemu-options.hx has docs for each backend object type. We should
add rng-builtin there, and also perhaps update rng-random docs to
have it encourse use of rng-builtin by default instead.

The code itself looks good.

> 
> diff --git a/backends/Makefile.objs b/backends/Makefile.objs
> index ff619d31b461..8da4a508d97b 100644
> --- a/backends/Makefile.objs
> +++ b/backends/Makefile.objs
> @@ -1,4 +1,4 @@
> -common-obj-y += rng.o rng-egd.o
> +common-obj-y += rng.o rng-egd.o rng-builtin.o
>  common-obj-$(CONFIG_POSIX) += rng-random.o
>  
>  common-obj-$(CONFIG_TPM) += tpm.o
> diff --git a/backends/rng-builtin.c b/backends/rng-builtin.c
> new file mode 100644
> index 000000000000..b1264b745407
> --- /dev/null
> +++ b/backends/rng-builtin.c
> @@ -0,0 +1,56 @@
> +/*
> + * QEMU Builtin Random Number Generator Backend
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2 or later.
> + * See the COPYING file in the top-level directory.
> + */
> +
> +#include "qemu/osdep.h"
> +#include "sysemu/rng.h"
> +#include "qapi/error.h"
> +#include "qapi/qmp/qerror.h"
> +#include "qemu/main-loop.h"
> +#include "qemu/guest-random.h"
> +
> +#define TYPE_RNG_BUILTIN "rng-builtin"
> +#define RNG_BUILTIN(obj) OBJECT_CHECK(RngBuiltin, (obj), TYPE_RNG_BUILTIN)
> +
> +typedef struct RngBuiltin {
> +    RngBackend parent;
> +} RngBuiltin;
> +
> +static void rng_builtin_request_entropy(RngBackend *b, RngRequest *req)
> +{
> +    RngBuiltin *s = RNG_BUILTIN(b);
> +
> +    while (!QSIMPLEQ_EMPTY(&s->parent.requests)) {
> +        RngRequest *req = QSIMPLEQ_FIRST(&s->parent.requests);
> +
> +        qemu_guest_getrandom_nofail(req->data, req->size);
> +
> +        req->receive_entropy(req->opaque, req->data, req->size);
> +
> +        rng_backend_finalize_request(&s->parent, req);
> +    }
> +}
> +
> +static void rng_builtin_class_init(ObjectClass *klass, void *data)
> +{
> +    RngBackendClass *rbc = RNG_BACKEND_CLASS(klass);
> +
> +    rbc->request_entropy = rng_builtin_request_entropy;
> +}
> +
> +static const TypeInfo rng_builtin_info = {
> +    .name = TYPE_RNG_BUILTIN,
> +    .parent = TYPE_RNG_BACKEND,
> +    .instance_size = sizeof(RngBuiltin),
> +    .class_init = rng_builtin_class_init,
> +};
> +
> +static void register_types(void)
> +{
> +    type_register_static(&rng_builtin_info);
> +}
> +
> +type_init(register_types);
> -- 
> 2.20.1
> 

Regards,
Daniel

On 5/9/19 7:23 AM, Laurent Vivier wrote:
> Add a new RNG backend using QEMU builtin getrandom function.
> 
> It can be created with "-object rng-builtin".
> 
> This patch applies on top of
> "[PATCH v4 00/24] Add qemu_getrandom and ARMv8.5-RNG etc"
> Based-on: <20190506173353.32206-1-richard.henderson@linaro.org>
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  backends/Makefile.objs |  2 +-
>  backends/rng-builtin.c | 56 ++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 57 insertions(+), 1 deletion(-)
>  create mode 100644 backends/rng-builtin.c

Looks good.  Thanks for picking this up.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

> +static void rng_builtin_request_entropy(RngBackend *b, RngRequest *req)
> +{
> +    RngBuiltin *s = RNG_BUILTIN(b);
> +
> +    while (!QSIMPLEQ_EMPTY(&s->parent.requests)) {
> +        RngRequest *req = QSIMPLEQ_FIRST(&s->parent.requests);
> +
> +        qemu_guest_getrandom_nofail(req->data, req->size);
> +
> +        req->receive_entropy(req->opaque, req->data, req->size);
> +
> +        rng_backend_finalize_request(&s->parent, req);
> +    }
> +}

As an aside, with all of the callbacks involved, does anyone know if this gets
processed on the same thread as issued the cpu that issued the i/o operation?

The question is only relevant to debugging mode (w/ -seed), in that if we're
processing this from an i/o worker thread we won't have completely
deterministic results, as we may compete with other cpus for the same RNG.

There's probably not much that can reasonably be done if this does cross
threads, and certainly it would not affect normal usage, but I just wondered.


r~

On Thu, May 09, 2019 at 04:23:42PM +0200, Laurent Vivier wrote:
> Add a new RNG backend using QEMU builtin getrandom function.
> 
> It can be created with "-object rng-builtin".

Can you please specify a full, working example in the commit message?
It would be useful for later reference.

> This patch applies on top of
> "[PATCH v4 00/24] Add qemu_getrandom and ARMv8.5-RNG etc"
> Based-on: <20190506173353.32206-1-richard.henderson@linaro.org>
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  backends/Makefile.objs |  2 +-
>  backends/rng-builtin.c | 56 ++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 57 insertions(+), 1 deletion(-)
>  create mode 100644 backends/rng-builtin.c

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