Re: ARMv8.5 RNG driver (was :Re: [PATCH] virtio: rng: gracefully handle 0 byte returns)

[Andre Przywara <andre.przywara@arm.com>]

On Sat, 11 Nov 2023 20:08:36 -0700
Simon Glass <sjg@chromium.org> wrote:

Hi,

> On Fri, 10 Nov 2023 at 07:16, Andre Przywara <andre.przywara@arm.com> wrote:
> >
> > On Fri, 10 Nov 2023 05:53:59 -0700
> > Simon Glass <sjg@chromium.org> wrote:
> >
> > Hi Simon,
> >  
> > > On Tue, 7 Nov 2023 at 09:09, Andre Przywara <andre.przywara@arm.com> wrote:  
> > > >
> > > > According to the virtio v1.x "entropy device" specification, a virtio-rng
> > > > device is supposed to always return at least one byte of entropy.
> > > > However the virtio v0.9 spec does not mention such a requirement.
> > > >
> > > > The Arm Fixed Virtual Platform (FVP) implementation of virtio-rng always
> > > > returns 8 bytes less of entropy than requested. If 8 bytes or less are
> > > > requested, it will return 0 bytes.
> > > > This behaviour makes U-Boot's virtio_rng_read() implementation go into an
> > > > endless loop, hanging the system.
> > > >
> > > > Work around this problem by always requesting 8 bytes more than needed,
> > > > but only if a previous call to virtqueue_get_buf() returned 0 bytes.
> > > >
> > > > This should never trigger on a v1.x spec compliant implementation, but
> > > > fixes the hang on the Arm FVP.
> > > >
> > > > Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> > > > Reported-by: Peter Hoyes <peter.hoyes@arm.com>
> > > > ---
> > > >  drivers/virtio/virtio_rng.c | 9 +++++++--
> > > >  1 file changed, 7 insertions(+), 2 deletions(-)  
> > >
> > > Unrelated to this patch, but do you know the hardware architecture of
> > > the ARM RNG? Is there one RNG unit per CPU or one for the whole SoC?  
> >
> > Architecturally and from a software perspective the ARMv8.5 FEAT_RNG
> > feature is a system register, so per-core. Theoretically the availability
> > could differ between cores, but the CPU ID feature registers are also
> > per-core, so as long as we run on a single core, or always at least
> > read from the same core, it's all good.
> >
> > Now the architecture only describes the CPU instruction aspect of the
> > feature, and establishes rules for the quality and spec conformance, but
> > leaves the actual source of the entropy open to the integrator.
> >
> > The manual in the Neoverse V1 core[1] (still not a SoC!) states that the
> > actual entropy source is a memory mapped peripheral, its address being
> > held in an internal, per-core register. So you can have one shared entropy
> > source per SoC, or a private instance for each core, that's up to the
> > actual integrator to design.
> >
> > From the software perspective this shouldn't matter, though: the feature
> > is "per-core", how this is backed is an implementation detail.  
> 
> Would it make sense to model this as a memory-mapped peripheral under
> /soc (perhaps one without an address?)

No, absolutely not. What I mentioned above is a somewhat hidden
implementation detail of that *particular core*. One big reason for
having those architected *system registers* is to do away with all
those implementation specific ways to access an entropy source, and
make this dead easy for software (including userland!) to use it:
Check the CPU ID register, read the sysreg. No prior knowledge required.

I now deeply regret sending this Armv8.5 RNG driver. I have an
alternative solution, just got distracted later this week to finish
this up.
Let's have a discussion there, or we find a way to probe UCLASS_RNG
drivers other than through devicetree nodes. If U-Boot really insists on
matching drivers to DT nodes 1:1, that's a really limiting design
decision, and we should not proliferate this by shoehorning everyone
and their dog into devicetree.

Cheers,
Andre

> > [1]
> > https://developer.arm.com/documentation/101427/0102/Functional-description/Random-number-support/About-the-random-number-support
> >
> >  
> > > > diff --git a/drivers/virtio/virtio_rng.c b/drivers/virtio/virtio_rng.c
> > > > index b85545c2ee5..786359a6e36 100644
> > > > --- a/drivers/virtio/virtio_rng.c
> > > > +++ b/drivers/virtio/virtio_rng.c
> > > > @@ -20,7 +20,7 @@ struct virtio_rng_priv {
> > > >  static int virtio_rng_read(struct udevice *dev, void *data, size_t len)
> > > >  {
> > > >         int ret;
> > > > -       unsigned int rsize;
> > > > +       unsigned int rsize = 1;
> > > >         unsigned char buf[BUFFER_SIZE] __aligned(4);
> > > >         unsigned char *ptr = data;
> > > >         struct virtio_sg sg;
> > > > @@ -29,7 +29,12 @@ static int virtio_rng_read(struct udevice *dev, void *data, size_t len)
> > > >
> > > >         while (len) {
> > > >                 sg.addr = buf;
> > > > -               sg.length = min(len, sizeof(buf));
> > > > +               /*
> > > > +                * Work around implementations which always return 8 bytes
> > > > +                * less than requested, down to 0 bytes, which would
> > > > +                * cause an endless loop otherwise.
> > > > +                */
> > > > +               sg.length = min(rsize ? len : len + 8, sizeof(buf));
> > > >                 sgs[0] = &sg;
> > > >
> > > >                 ret = virtqueue_add(priv->rng_vq, sgs, 0, 1);
> > > > --
> > > > 2.25.1  
> 
> Regards,
> Simon