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

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Date:   Thu, 12 Dec 2019 13:13:29 +0800
From:   Neal Liu <neal.liu@...iatek.com>
To:     Marc Zyngier <maz@...nel.org>
CC:     Florian Fainelli <f.fainelli@...il.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        <pawel.moll@....com>, Mark Rutland <mark.rutland@....com>,
        DTML <devicetree@...r.kernel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        wsd_upstream <wsd_upstream@...iatek.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Sean Wang <sean.wang@...nel.org>,
        <linux-mediatek@...ts.infradead.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        <linux-crypto@...r.kernel.org>, Matt Mackall <mpm@...enic.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Crystal Guo (郭晶) 
        <crystal.guo@...iatek.com>, "Will Deacon" <will@...nel.org>,
        Lars Persson <lists@...h.nu>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v5 3/3] hwrng: add mtk-sec-rng driver

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@...aro.org> wrote:
> >>
> >>> (adding some more arm64 folks)
> >>>
> >>> On Fri, 29 Nov 2019 at 11:30, Neal Liu <neal.liu@...iatek.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@...iatek.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.

Thanks

-Neal