Re: [PATCH v2 1/2] KVM: arm64: mixed-width check should be skipped for uninitialized vCPUs

From: Marc Zyngier <maz@kernel.org>
To: Reiji Watanabe <reijiw@google.com>
Cc: kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	James Morse <james.morse@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Will Deacon <will@kernel.org>, Peter Shier <pshier@google.com>,
	Ricardo Koller <ricarkol@google.com>,
	Oliver Upton <oupton@google.com>,
	Jing Zhang <jingzhangos@google.com>,
	Raghavendra Rao Anata <rananta@google.com>
Subject: Re: [PATCH v2 1/2] KVM: arm64: mixed-width check should be skipped for uninitialized vCPUs
Date: Wed, 09 Feb 2022 12:04:37 +0000	[thread overview]
Message-ID: <875ypo5jqi.wl-maz@kernel.org> (raw)
In-Reply-To: <CAAeT=FwjcgTM0hKSERfVMYDvYWqdC+Deqd=x2xT=-Zymb6SLtA@mail.gmail.com>

Hi Reiji,

On Wed, 09 Feb 2022 05:32:36 +0000,
Reiji Watanabe <reijiw@google.com> wrote:
> 
> Hi Marc,
> 
> On Tue, Feb 8, 2022 at 6:41 AM Marc Zyngier <maz@kernel.org> wrote:
> >
> > In [1], I suggested another approach that didn't require extra state,
> > and moved the existing checks under the kvm lock. What was wrong with
> > that approach?
> 
> With that approach, even for a vcpu that has a broken set of features,
> which leads kvm_reset_vcpu() to fail for the vcpu, the vcpu->arch.features
> are checked by other vCPUs' vcpu_allowed_register_width() until the
> vcpu->arch.target is set to -1.
> Due to this, I would think some or possibly all vCPUs' kvm_reset_vcpu()
> may or may not fail (e.g. if userspace tries to configure vCPU#0 with
> 32bit EL1, and vCPU#1 and #2 with 64 bit EL1, KVM_ARM_VCPU_INIT
> for either vCPU#0, or both vCPU#1 and #2 should fail.  But, with that
> approach, it doesn't always work that way.  Instead, KVM_ARM_VCPU_INIT
> for all vCPUs could fail or KVM_ARM_VCPU_INIT for vCPU#0 and #1 could
> fail while the one for CPU#2 works).
> Also, even after the first KVM_RUN for vCPUs are already done,
> (the first) KVM_ARM_VCPU_INIT for another vCPU could cause the
> kvm_reset_vcpu() for those vCPUs to fail.
> 
> I would think those behaviors are odd, and I wanted to avoid them.

OK, fair enough. But then you need to remove most of the uses of
KVM_ARM_VCPU_EL1_32BIT so that it is only used as a userspace
interface and maybe not carried as part of the vcpu feature flag
anymore.

Also, we really should turn all these various bits in the kvm struct
into a set of flags. I have a patch posted there[1] for this, feel
free to pick it up.

Thanks,

	M.

[1] https://lore.kernel.org/r/20211004174849.2831548-2-maz@kernel.org

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

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