Re: [PATCH v5 3/3] hwrng: add mtk-sec-rng driver

[Marc Zyngier]

On 2019-12-12 14:03, Ard Biesheuvel wrote:
> On Thu, 12 Dec 2019 at 12:45, Marc Zyngier <maz@xxxxxxxxxx> wrote:
> > On 2019-12-12 05:13, Neal Liu wrote:
> > > On Tue, 2019-12-03 at 11:17 +0000, Marc Zyngier wrote:
> > >> On 2019-12-03 04:16, Florian Fainelli wrote:
> > >> > On 12/2/2019 11:11 AM, Marc Zyngier wrote:
> > >> >> On Mon, 2 Dec 2019 16:12:09 +0000
> > >> >> Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
> > >> >>
> > >> >>> (adding some more arm64 folks)
> > >> >>>
> > >> >>> On Fri, 29 Nov 2019 at 11:30, Neal Liu 
> > <neal.liu@xxxxxxxxxxxx>
> > >> >>> wrote:
> > >> >>>>
> > >> >>>> On Fri, 2019-11-29 at 18:02 +0800, Lars Persson wrote:
> > >> >>>>> Hi Neal,
> > >> >>>>>
> > >> >>>>> On Wed, Nov 27, 2019 at 3:23 PM Neal Liu
> > >> <neal.liu@xxxxxxxxxxxx>
> > >> >>>>> wrote:
> > >> >>>>>>
> > >> >>>>>> For MediaTek SoCs on ARMv8 with TrustZone enabled,
> > >> peripherals
> > >> >>>>>> like
> > >> >>>>>> entropy sources is not accessible from normal world 
> > (linux)
> > >> and
> > >> >>>>>> rather accessible from secure world (ATF/TEE) only. This
> > >> driver
> > >> >>>>>> aims
> > >> >>>>>> to provide a generic interface to ATF rng service.
> > >> >>>>>>
> > >> >>>>>
> > >> >>>>> I am working on several SoCs that also will need this kind 
> > of
> > >> >>>>> driver
> > >> >>>>> to get entropy from Arm trusted firmware.
> > >> >>>>> If you intend to make this a generic interface, please 
> > clean
> > >> up
> > >> >>>>> the
> > >> >>>>> references to MediaTek and give it a more generic name. For
> > >> >>>>> example
> > >> >>>>> "Arm Trusted Firmware random number driver".
> > >> >>>>>
> > >> >>>>> It will also be helpful if the SMC call number is
> > >> configurable.
> > >> >>>>>
> > >> >>>>> - Lars
> > >> >>>>
> > >> >>>> Yes, I'm trying to make this to a generic interface. I'll 
> > try
> > >> to
> > >> >>>> make
> > >> >>>> HW/platform related dependency to be configurable and let it
> > >> more
> > >> >>>> generic.
> > >> >>>> Thanks for your suggestion.
> > >> >>>>
> > >> >>>
> > >> >>> I don't think it makes sense for each arm64 platform to 
> > expose
> > >> an
> > >> >>> entropy source via SMC calls in a slightly different way, and
> > >> model
> > >> >>> it
> > >> >>> as a h/w driver. Instead, we should try to standardize this, 
> > and
> > >> >>> perhaps expose it via the architectural helpers that already
> > >> exist
> > >> >>> (get_random_seed_long() and friends), so they get plugged 
> > into
> > >> the
> > >> >>> kernel random pool driver directly.
> > >> >>
> > >> >> Absolutely. I'd love to see a standard, ARM-specified,
> > >> virtualizable
> > >> >> RNG that is abstracted from the HW.
> > >> >
> > >> > Do you think we could use virtio-rng on top of a modified
> > >> virtio-mmio
> > >> > which instead of being backed by a hardware mailbox, could use
> > >> > hvc/smc
> > >> > calls to signal writes to shared memory and get notifications 
> > via
> > >> an
> > >> > interrupt? This would also open up the doors to other virtio 
> > uses
> > >> > cases
> > >> > beyond just RNG (e.g.: console, block devices?). If this is
> > >> > completely
> > >> > stupid, then please disregard this comment.
> > >>
> > >> The problem with a virtio device is that it is a ... device. What 
> > we
> > >> want
> > >> is to be able to have access to an entropy source extremely early 
> > in
> > >> the
> > >> kernel life, and devices tend to be available pretty late in the
> > >> game.
> > >> This means we cannot plug them in the architectural helpers that 
> > Ard
> > >> mentions above.
> > >>
> > >> What you're suggesting looks more like a new kind of virtio
> > >> transport,
> > >> which is interesting, in a remarkably twisted way... ;-)
> > >>
> > >> Thanks,
> > >>
> > >>          M.
> > >
> > > In conclusion, is it helpful that hw_random has a generic 
> > interface
> > > to
> > > add device randomness by talking to hwrng which is implemented in 
> > the
> > > firmware or the hypervisor?
> > > For most chip vendors, I think the answer is yes. We already 
> > prepared
> > > a
> > > new patchset and need you agree with this idea.
> >
> > As long as it is a *unified* interface, I'm all for that.
>
>
> Yeah, but I'm not sure it makes sense to model it as a device like
> this. It would be nice if we could tie this into the ARM SMCCC
> discovery, and use the SMC calls to back arch_get_random_seed_long()

Probably I wasn't clear enough, but that's really what I meant by
a unified interface (implemented by the firmware or the hypervisor).

> [provided we fix the braindead way in which that is being used today
> in the interrupt code]

Ah, I said I'd look into it. Thanks for the reminder...

Thanks,

        M.
--
Jazz is not dead. It just smells funny...