Re: [PATCH 1/3] hw/misc: Introduce AMD/Xilix Versal TRNG device

[Peter Maydell]

On Fri, 1 Sept 2023 at 05:16, Ho, Tong <tong.ho@amd.com> wrote:
>
> Hi Richard,
>
> Thanks for your input.
>
> I have questions regarding using qemu/guest-random.h for QEMU device models.
>
> Using qemu/guest-random.h, how can this TRNG model ensure its independence 
> from
> other uses of the same qemu_guest_getrandom() and qemu_guest_random_seed_*()?
>
> By "other uses", I mean components and/or devices using qemu/guest-random.h 
> but unrelated to this Xilinx Versal TRNG device.
>
> By "independent", I mean the Xilinx Versal TRNG device is:
>
> 1. Not impacted by other uses that may or may not need to set the '-seed' 
> option, and
>
> 2. Not impacting other uses just because a Xilinx Versal machine user decides 
> to use deterministic mode *only" for this TRNG device.
>
> Also, I am at a loss in how unrelated QEMU devices can remain independent 
> when:
>
> 3. qemu/guest-random.h uses '__thread' variable for GRand context, but
>
> 4. QEMU devices run mostly as co-routines and not as separate threads.

You shouldn't need to care about any of this. Just assume you can
get decent quality random numbers from qemu_guest_getrandom() or
qemu_guest_getrandom_nofail(). The -seed option is for the
entire simulation, not specific to individual RNG devices.

> I suppose the Versal TRNG implementation could use g_rand_*() directly,
> having a GRand object in the device state and seeding through 
> g_rand_set_seed_array().

Don't do something non-standard. Write this RNG device the same way
we do all other RNG devices in QEMU.

thanks
-- PMM