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

[Florian Fainelli]

On 8/15/2021 11:27 AM, Marc Zyngier wrote:
> On Sun, 15 Aug 2021 08:28:47 +0100,
> Florian Fainelli <f.fainelli at gmail.com> wrote:
>>
>>
>>
>> On 8/12/2021 9:02 PM, Marc Zyngier wrote:
>>> 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.

> 
>>
>>>
>>> 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