Re: [PATCH v1] random: block in /dev/urandom

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Hey Arnd/Guenter,

On Wed, Mar 23, 2022 at 4:53 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Wed, Mar 23, 2022 at 3:23 PM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> >
> > On 3/23/22 05:10, Mark Brown wrote:
> > > On Tue, Mar 22, 2022 at 02:54:20PM -0700, Guenter Roeck wrote:
> > > Kind of academic given that Jason seems to have a handle on what the
> > > issues are but for KernelCI it's variations on mach-virt, plus
> > > versatile-pb.  There's a physical cubietruck as well, and BeagleBone
> > > Blacks among others.  My best guess would be systems with low RAM are
> > > somehow more prone to issues.
> >
> > I don't think it is entirely academic. versatile-pb fails for me;
> > if it doesn't fail at KernelCI, I'd like to understand why - not to
> > fix it in my test environment, but to make sure that I _don't_ fix it.
> > After all, it _is_ a regression. Even if that regression is triggered
> > by bad (for a given definition of "bad") userspace code, it is still
> > a regression.
>
> Maybe kernelci has a virtio-rng device assigned to the machine
> and you don't? That would clearly avoid the issue here.

Indeed it's probably something like that. Or maybe they're networked
with something that has a steady stream of interrupts. I say this
because I was able to reproduce Guenter's findings using the
versatilepb machine with the versatile_defconfig config and the
versatile-pb.dtb file. Indeed this board doesn't have a cycle counter.
However, I did have success using the fallback timer and the other
patches in the jd/for-guenter branch, so at least for versatile's
nuances, I think (hope?) there's a reasonable success story here.

Jason



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