Re: [PATCH v2 3/3] arm64: Allow nVHE impaired CPUs to boot without CONFIG_ARM64_VHE

From: Will Deacon <will@kernel.org>
To: Marc Zyngier <maz@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org,
	Hector Martin <marcan@marcan.st>, Arnd Bergmann <arnd@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	kernel-team@android.com
Subject: Re: [PATCH v2 3/3] arm64: Allow nVHE impaired CPUs to boot without CONFIG_ARM64_VHE
Date: Thu, 8 Apr 2021 11:58:23 +0100	[thread overview]
Message-ID: <20210408105823.GB17280@willie-the-truck> (raw)
In-Reply-To: <8474e539259346842178f51a26d59d74@kernel.org>

On Thu, Apr 08, 2021 at 11:31:42AM +0100, Marc Zyngier wrote:
> On 2021-04-07 22:18, Will Deacon wrote:
> > On Tue, Mar 30, 2021 at 06:39:47PM +0100, Marc Zyngier wrote:
> > > CPUs stuck in VHE mode need some additional care if the kernel
> > > is compiled without CONFIG_ARM64_VHE.
> > > 
> > > Treat this case as another version of a mismatched boot, and
> > > prevent KVM from being initialised. The machine will boot in
> > > some bizarre state, using TPIDR_EL1 instead of TPIDR_EL2, but
> > > otherwise be functional.
> > > 
> > > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > > ---
> > >  arch/arm64/include/asm/virt.h | 18 +++++++++++++-----
> > >  arch/arm64/kvm/va_layout.c    |  9 +++++++++
> > >  2 files changed, 22 insertions(+), 5 deletions(-)
> > 
> > Hmm, I think we definitely need _something_ here, but it's a bit
> > annoying
> > to put ourselves into this weird state just for the sake of one stupid
> > machine.
> 
> Which is why I'm not keen at all on this patch, and I'm happy to see
> the machine die a painful death. We really can't be blamed for terminally
> buggy HW, which the M1 obviously is.

Perhaps, but it's not clear at all how the problem manifests and so we
_will_ be blamed when things go wonky, especially as the real culprits
all seem to be hiding...

> > What if we dropped CONFIG_ARM64_VHE and made the VHE code unconditional
> > instead? Is there a good reason to allow it to be disabled nowadays?
> 
> What do we do for the other camp, aka people really wanting to run nVHE
> without any command line parameter? I can't see why you'd want to do
> that, but hey, that's only me.

They can pass the command-line parameter, no? If we ever get CMDLINE_EXTEND
support back, they could even add it in there!

> I'd be quite happy to see CONFIG_ARM64_VHE go though. Let me know if you
> want a patch doing that instead.

Yes please.

Will

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