Re: [PATCH 1/2] KVM: arm64: Disable KVM on systems with a VPIPT i-cache

From: Marc Zyngier <maz@kernel.org>
To: Will Deacon <will@kernel.org>,
		Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org,
	kvm@vger.kernel.org, Catalin Marinas <catalin.marinas@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	James Morse <james.morse@arm.com>,
	Oliver Upton <oliver.upton@linux.dev>,
	Zenghui Yu <yuzenghui@huawei.com>
Subject: Re: [PATCH 1/2] KVM: arm64: Disable KVM on systems with a VPIPT i-cache
Date: Fri, 20 Jan 2023 11:49:17 +0000	[thread overview]
Message-ID: <86sfg5mnnm.wl-maz@kernel.org> (raw)
In-Reply-To: <20230120101415.GA21784@willie-the-truck>

On Fri, 20 Jan 2023 10:14:16 +0000,
Will Deacon <will@kernel.org> wrote:
> 
> On Fri, Jan 13, 2023 at 05:25:22PM +0000, Marc Zyngier wrote:
> > Systems with a VMID-tagged PIPT i-cache have been supported for
> > a while by Linux and KVM. However, these systems never appeared
> > on our side of the multiverse.
> > 
> > Refuse to initialise KVM on such a machine, should then ever appear.
> > Following changes will drop the support from the hypervisor.
> > 
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> >  arch/arm64/kvm/arm.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 9c5573bc4614..508deed213a2 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -2195,6 +2195,11 @@ int kvm_arch_init(void *opaque)
> >  	int err;
> >  	bool in_hyp_mode;
> >  
> > +	if (icache_is_vpipt()) {
> > +		kvm_info("Incompatible VPIPT I-Cache policy\n");
> > +		return -ENODEV;
> > +	}
> 
> Hmm, does this work properly with late CPU onlining? For example, if my set
> of boot CPUs are all friendly PIPT and KVM initialises happily, but then I
> late online a CPU with a horrible VPIPT policy, I worry that we'll quietly
> do the wrong thing wrt maintenance.

Yup. The problem is what do we do in that case? Apart from preventing
the late onlining itself?

> 
> If that's the case, then arguably we already have a bug in the cases where
> we trap and emulate accesses to CTR_EL0 from userspace because I _think_
> we'll change the L1Ip field at runtime after userspace could've already read
> it.
> 
> Is there something that stops us from ended up in this situation?

Probably not. Userspace will observe the wrong thing, and this applies
to *any* late onlining with a more restrictive cache topology (such as
PIPT -> VIPT). Unclear how the trapping will be engaged on the *other*
CPUs as well...

I've tried to reverse-engineer the cpufeature arrays again, and failed
to find a good solution for this.

Suzuki, what do you think?

	M.

-- 
Without deviation from the norm, progress is not possible.

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