next/master bisection: baseline.login on rk3288-rock2-square

[Ard Biesheuvel]

On Thu, 4 Feb 2021 at 15:09, Russell King - ARM Linux admin
<linux at armlinux.org.uk> wrote:
>
> On Thu, Feb 04, 2021 at 12:26:44PM +0000, Marc Zyngier wrote:
> > I agree. With set/way CMOs, there is no way to reach the PoC if
> > it beyond the system cache, leading to an unbootable kernel.
> > This is actually pretty well documented in the architecture,
> > and it did bite us for the first time on XGene-1, 7 years ago.
>
> That may be, however we still do set/way maintenance to invalidate
> the L1 cache as that is required for ARMv7 to place the cache into
> a known state, as stated by the architecture reference manual.
>

Getting a certain cache at a certain level into a known state is a
valid use of set/way ops, and is simply unnecessary when running under
virtualization, but doesn't do any harm.

Pushing contents of the cache hierarchy to main memory is *not* a
valid use of set/way ops, and so there is no point in pretending that
set/way ops will produce the same results as by-VA ops. Only the by-VA
ops give the architectural guarantees that we rely on for correctness.

> Arguably, that should be done by firmware, but when starting
> secondary CPUs, there are platforms out there which do not bring
> the L1 cache to a defined state. So we are pretty much stuck with
> doing set/way operations during CPU initialisation in the main
> kernel.
>

Indeed. And this is unfortunate, but not the end of the world.

> If ARMv8 decides that this is not supportable, then that's a matter
> for ARMv8 to address without impacting the requirements of ARMv7.
>

I'm not sure what you mean here. The v7 architecture is crystal clear
about the difference between set/way ops (managing a single cache),
and by-VA ops (managing the 'cachedness' state of a memory region).
The semantics are radically different, regardless of v7 vs v8 or
AArch32 vs AArch64.