linux-kernel - Re: [PATCH v4 23/23] arm64: Panic when VHE and non VHE CPUs coexist

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Date:	Mon, 15 Feb 2016 17:26:53 +0000
From:	Will Deacon <will.deacon@....com>
To:	Marc Zyngier <marc.zyngier@....com>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Mark Rutland <mark.rutland@....com>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu
Subject: Re: [PATCH v4 23/23] arm64: Panic when VHE and non VHE CPUs coexist

On Thu, Feb 11, 2016 at 06:40:04PM +0000, Marc Zyngier wrote:
> Having both VHE and non-VHE capable CPUs in the same system
> is likely to be a recipe for disaster.
> 
> If the boot CPU has VHE, but a secondary is not, we won't be
> able to downgrade and run the kernel at EL1. Add CPU hotplug
> to the mix, and this produces a terrifying mess.
> 
> Let's solve the problem once and for all. If you mix VHE and
> non-VHE CPUs in the same system, you deserve to loose, and this
> patch makes sure you don't get a chance.
> 
> This is implemented by storing the kernel execution level in
> a global variable. Secondaries will park themselves in a
> WFI loop if they observe a mismatch. Also, the primary CPU
> will detect that the secondary CPU has died on a mismatched
> execution level. Panic will follow.

This should really be based on Suzuki's series for handling generic
mismatches:

http://lists.infradead.org/pipermail/linux-arm-kernel/2016-January/401727.html

To avoid growing a dependency on something that's unlikely to make it
for 4.6, I'd be inclined to drop your homegrown checks altogether amd
help Suzuki with his series as a separate activity (i.e. it needn't be
a blocker imo).

Will