Re: [PATCH 0/5] arm64: Survival kit for SCR_EL3.HCE==0 conditions

On 8/15/2021 11:27 AM, Marc Zyngier wrote:

On Sun, 15 Aug 2021 08:28:47 +0100,
Florian Fainelli [off-list ref] wrote:

quoted

On 8/12/2021 9:02 PM, Marc Zyngier wrote:

quoted

Anyone vaguely familiar with the ARMv8 architecture would quickly
understand that entering the kernel at EL2 without enabling the HVC
instruction is... living dangerously. But as it turns out [0], there
is a whole range of (*cough*) "high quality" (*cough*) Broadcom
systems out there configured exactly like that.

Some Broadcom systems, namely the 4908 and all of those using CFE,
they later switched to u-boot and ATF and got it right.

Do we have a list of the affected systems?

That is going to be hard to come up with given that OEMs such as Asus, 
TP-Link, or Netgear and multiple devices are affected. I would start 
with this list which is far from exhaustive: Netgear R8000P, TP-Link 
Archer C2300, Asus GT AC5300.

quoted

quoted

If you are speechless, I'm right with you.

These machines have stopped being able to boot an upstream kernel
since 5.12, where we changed the way we switch from nVHE to VHE, as
this relies on the HVC instruction being usable... It is also worth
noting that these systems have never been able to use KVM. Or kexec.

This small series addresses the issue by detecting an UNDEFing HVC in
a fairly controlled environment, and in this case pretend that we have
booted at EL1. It also documents the requirement for SCR_EL3.HCE to be
set to *1* if the kernel is entered at EL2. Turns out that we really
have to state the obvious.

This has been tested on a FVP model with a hacked-up boot-wrapper.

Note that I really don't think any of this is -stable material, except
maybe for the documentation. It isn't 5.14 material either. Best case,
this is 5.15, or maybe even later. If ever.

While I am very appreciative of the work you have done here to try to
get the dysfunctional systems to warn and continue to boot, I would
rather we try to load a minimal shim at EL3 capable of fixing up any
incorrect EL3 register setting ahead of loading the kernel provided
this is possible at all on a commercially available system.

That would be the best thing to do, and would make the machine fully
usable. I still think we need to have something in the kernel to at
least let the user know that their system is misconfigured though.

If CFE allows a payload to be loaded at EL3 and executed on all CPUs,
that would be absolutely awesome. It would even allow switching over
to ATF...

Supposedly there are GPL tarballs containing an u-boot that is suitable 
for the 4908, however I have no idea where to find those. In premise you 
could chainload those to CFE and a get seemingly functional system.
-- 
Florian

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