linux-kernel - Re: [PATCH 0/5] arm64: Survival kit for SCR

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Message-ID: <7ffa35bb-1f2c-19ff-fe4b-33267fba80e8@gmail.com>
Date:   Sun, 15 Aug 2021 09:28:47 +0200
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Marc Zyngier <maz@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     Rafał Miłecki <zajec5@...il.com>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        bcm-kernel-feedback-list@...adcom.com, kernel-team@...roid.com
Subject: 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