linux-kernel - Re: [PATCH v3 06/23] kvm: arm64: Add kvm-arm.protected early kernel parameter

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Date:   Tue, 1 Dec 2020 14:43:49 +0000
From:   David Brazdil <dbrazdil@...gle.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Sudeep Holla <sudeep.holla@....com>, kvmarm@...ts.cs.columbia.edu,
        Jonathan Corbet <corbet@....net>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kernel-team@...roid.com
Subject: Re: [PATCH v3 06/23] kvm: arm64: Add kvm-arm.protected early kernel
 parameter

> > > be just me, but if you agree please update so that it doesn't give remote
> > > idea that it is not valid on VHE enabled hardware.
> > > 
> > > I was trying to run this on the hardware and was trying to understand the
> > > details on how to do that.
> > 
> > I see what you're saying, but !CONFIG_ARM64_VHE isn't accurate either. The
> > option makes sense if:
> >   1) all cores booted in EL2
> >      == is_hyp_mode_available()
> >   2) ID_AA64MMFR1_EL1.VH=0 or !CONFIG_ARM64_VHE
> >      == !is_kernel_in_hyp_mode()
> > 
> > The former feels implied for KVM, the latter could be 'Valid if the kernel
> > is running in EL1'? WDYT?
> 
> I reckon we can avoid the restriction if we instead add an early stub
> like with have for KASLR. That way we could parse the command line
> early, and if necessary re-initialize EL2 and drop to EL1 before the
> main kernel has to make any decisions about how to initialize things.
> That would allow us to have a more general kvm-arm.mode option where a
> single kernel Image could support:
> 
> * "protected" mode on nVHE or VHE HW
> * "nvhe" mode on nVHE or VHE HW
> * "vhe" mode on VHE HW
> 
> ... defaulting to VHE/nVHE modes depending on HW support.
> 
> That would also be somewhat future-proof if we have to add other
> variants of protected mode in future, as we could extend the mode option
> with parameters for each mode.

Agreed that 'mode' is a more future-proof flag and I would very much love to
have an option to force nVHE on VHE HW. I however expect that the early stub
would not be a trivial addition and would not want to get into that in this
series. Could we agree on 'protected' as the only supported value for the time
being?

David