[PATCH v3 0/2] rng: Provide a RNG based on the RISC-V Zkr ISA extension

[Simon Glass]

Hi Andre,

On Mon, 6 Nov 2023 at 10:26, Andre Przywara <andre.przywara at arm.com> wrote:
>
> On Sat, 4 Nov 2023 19:45:06 +0000
> Simon Glass <sjg at chromium.org> wrote:
>
> Hi,
>
> > On Sat, 4 Nov 2023 at 17:13, Andre Przywara <andre.przywara at arm.com> wrote:
> > >
> > > On Fri, 3 Nov 2023 13:38:58 -0600
> > > Simon Glass <sjg at chromium.org> wrote:
> > >
> > > Hi Simon,
> > >
> > > > Hi Heinrich,
> > > >
> > > > On Wed, 1 Nov 2023 at 14:20, Heinrich Schuchardt
> > > > <heinrich.schuchardt at canonical.com> wrote:
> > > > >
> > > > > On 11/1/23 19:05, Andre Przywara wrote:
> > > > > > On Tue, 31 Oct 2023 14:55:50 +0200
> > > > > > Heinrich Schuchardt <heinrich.schuchardt at canonical.com> wrote:
> > > > > >
> > > > > > Hi Heinrich,
> > > > > >
> > > > > >> The Zkr ISA extension (ratified Nov 2021) introduced the seed CSR. It
> > > > > >> provides an interface to a physical entropy source.
> > > > > >>
> > > > > >> A RNG driver based on the seed CSR is provided. It depends on
> > > > > >> mseccfg.sseed being set in the SBI firmware.
> > > > > >
> > > > > > As you might have seen, I added a similar driver for the respective Arm
> > > > > > functionality:
> > > > > > https://lore.kernel.org/u-boot/20230830113230.3925868-1-andre.przywara@arm.com/
> > > > > >
> > > > > > And I see that you seem to use the same mechanism to probe and init the
> > > > > > driver: U_BOOT_DRVINFO and fail in probe() if the feature is not
> > > > > > implemented.
> > > > > > One downside of this approach is that the driver is always loaded (and
> > > > > > visible in the DM tree), even with the feature not being available.
> > > > > > That doesn't seem too much of a problem on the first glance, but it
> > > > > > occupies a device number, and any subsequent other DM_RNG devices
> > > > > > (like virtio-rng) typically get higher device numbers. So without
> > > > > > the feature, but with virtio-rng, I get:
> > > > > > VExpress64# rng 0
> > > > > > No RNG device
> > > >
> > > > Why do we get this? If the device is not there, the bind() function
> > > > can return -ENODEV
> > > >
> > > > I see this in U-Boot:
> > > >
> > > > U_BOOT_DRVINFO(cpu_arm_rndr) = {
> > > >
> > > > We should not use this.
> > >
> > > Agreed.
> > >
> > > > Use the devicetree.
> > >
> > > No, this is definitely not something for the DT, at least not on ARM.
> > > It's perfectly discoverable via the architected CPU ID registers.
> > > Similar to PCI and USB devices, which we don't probe via the DT as well.
> > >
> > > It's arguably not proper "driver" material per se, as I've argued before, but
> > > it's the simplest solution and fits in nicely otherwise.
> > >
> > > I was wondering if it might be something for UCLASS_CPU, something like
> > > a "CPU feature bus": to let devices register on one on the many CPU
> > > features (instead of compatible strings), then only bind() those
> > > drivers it the respective bit is set.
> > >
> > > Does that make sense? Would that be doable without boiling the ocean?
> > > As I don't know if we see many users apart from this.
> >
> > I have seen this so many times, where people want to avoid putting
> > things in the DT and then are surprised that everything is difficult,
> > broken and confusing. Why not just follow the rules? It is not just
> > about whether we can avoid it, etc. It is about how devices fit
> > together cohesively in the system, and how U-Boot operates.
>
> A devicetree is only for peripherals *that cannot be located by probing*.

I have to stop you there. It absolutely is not limited to that.

> Which are traditionally most peripherals in non-server Arm SoCs. While I
> do love the DT, the best DT node is the one you don't need.

We need it in U-Boot, at least.

I'll send a patch with a warning on U_BOOT_DRVINFO() as it seems that
some people did not see the header-file comment.

Let's just stop this discussion and instead talk about the binding we need.

>
> But as Heinrich also said: those instructions are not peripherals, they
> are part of an instruction set extensions, the same story as with x86's
> RDRAND instruction. We don't have those in ACPI or so as well, because
> CPUID has you covered. The same on ARM, ID_AA64ISAR0_EL1 is readable on
> every chip (outside of EL0), and tells you whether you have the RNDR
> register or not. IIUC RISC-V is slightly different here, since not all ISA
> extensions are covered by CSRs, hence some of them indeed listed in the DT.
>
> So a proper solution(TM) would be to split this up in architectural
> *instructions* and proper TRNG *devices*, maybe wrapping this up in some
> function that tests both. This is roughly what the kernel does, somewhat
> abstracted by the concept of "entropy sources", which could be TRNG
> devices, CPU instructions, interrupt jitter or even "instruction execution
> jitter"[1], with the latter two definitely not being devices really at all.
>
> But I don't know if U-Boot wants to go through the hassle of this whole
> framework, as we tend to implement things much easier. But a simple
> get_cpu_random() function, implemented per architecture, and with some
> kind of success flag, should be easy enough to do. Then either the users
> (UEFI?) explicitly call this before trying UCLASS_RNG, or we wrap this for
> every RNG user.
>
> Cheers,
> Andre
>
> > > > > > VExpress64# rng 1
> > > > > > 00000000: f3 88 b6 d4 24 da 49 ca 49 f7 9e 66 5f 12 07 b2  ....$.I.I..f_...
> > > > >
> > > > >
> > > > > Essentially in any case were you have multiple drivers for the same
> > > > > device using uclass_get_device(, 0, ) and uclass_find_first_device()
> > > > > will only give you the first bound device and not the first successfully
> > > > > probed device. Furthermore neither of this functions causes probing.
> > > > > This is not restricted to the RNG drivers but could also happen with
> > > > > multiple TPM drivers or multiple watchdogs.
> > > > >
> > > > > This patch is related to the problem:
> > > > >
> > > > > [PATCH v1] rng: add dm_rng_read_default() helper
> > > > > https://lore.kernel.org/u-boot/4e28a388-f5b1-4cf7-b0e3-b12a876d0567@gmx.de/T/#me44263ec9141e3ea65ee232aa9a411fc6201bd95
> > > > >
> > > > > We have weak function platform_get_rng_device() which should be moved to
> > > > > drivers/rng/rng-uclass.c.
> > > > >
> > > > > We could add a function to drivers/core/uclass.c to retrieve the first
> > > > > successfully probed device. Another approach would be to implement
> > > > > uclass_driver.post_probe() in the RNG uclass to take note of the first
> > > > > successfully probed device.
> > > > >
> > > > > @Simon:
> > > > > What would make most sense from a DM design standpoint?
> > > >
> > > > I am sure I provided feedback on this at the time, but I don't
> > > > remember. OK I just found it here [1]. So the problem is entirely
> > > > because my feedback was not addressed. Please just address it and
> > > > avoid this sort of mess.
> > >
> > > Yeah, Tom just merged it, but that's not Heinrich's fault ;-)
> > >
> > > > So arm_rndr should have a devicetree compatible string and be bound
> > > > like anything else. If for some reason the device doesn't exist in the
> > > > hardware, it can return -ENODEV from its bind() method.
> > > >
> > > > If you want to control which RNG device is used for booting, you could
> > > > add a property to /bootstd with a phandle to the device. We are trying
> > > > to provide a standard approach to booting in U-Boot, used by all
> > > > methods. Doing one-off things for particular cases is best avoided.
> > >
> > > Picking the first usable device doesn't sound much like a one-off to me.
> > > After all the caller (be it UEFI or the rng command) later detect that
> > > this is not usable. So there might be some merit to cover this more
> > > automatically, either in the caller, or by providing a suitable wrapper
> > > function?
> >
> > Or just follow the existing mechanisms which have been in U-Boot for
> > years. Please...!
> >
> > [..]
> >
> >
> > Regards,
> > Simon
> >  >
> > > > [1] https://patchwork.ozlabs.org/project/uboot/patch/20230830113230.3925868-1-andre.przywara@arm.com/
> > >
>

Regards,
Simon