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To:     Neal Liu <neal.liu@mediatek.com>
Subject: Re: [PATCH v5 3/3] hwrng: add mtk-sec-rng driver
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Date:   Thu, 12 Dec 2019 11:45:56 +0000
From:   Marc Zyngier <maz@kernel.org>
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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.

Thanks,

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