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

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.

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. Rafal, is this 
something that CFE allows you to do (as I could not get a straight 
answer from that team), if so have you tried it?
-- 
Florian

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