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To:     Ard Biesheuvel <ard.biesheuvel@linaro.org>
Subject: Re: [PATCH v5 3/3] hwrng: add mtk-sec-rng driver
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Date:   Thu, 12 Dec 2019 14:30:38 +0000
From:   Marc Zyngier <maz@kernel.org>
Cc:     Neal Liu <neal.liu@mediatek.com>,
        Florian Fainelli <f.fainelli@gmail.com>,
        Pawel Moll <pawel.moll@arm.com>,
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On 2019-12-12 14:03, Ard Biesheuvel wrote:
> On Thu, 12 Dec 2019 at 12:45, Marc Zyngier <maz@kernel.org> 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@linaro.org> wrote:
>> >> >>
>> >> >>> (adding some more arm64 folks)
>> >> >>>
>> >> >>> On Fri, 29 Nov 2019 at 11:30, Neal Liu 
>> <neal.liu@mediatek.com>
>> >> >>> 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@mediatek.com>
>> >> >>>>> 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...