From: Oliver Upton <oliver.upton@linux.dev>
To: Marc Zyngier <maz@kernel.org>
Cc: kvmarm@lists.linux.dev, James Morse <james.morse@arm.com>,
Suzuki K Poulose <suzuki.poulose@arm.com>,
Zenghui Yu <yuzenghui@huawei.com>,
kvm@vger.kernel.org
Subject: Re: [PATCH] KVM: arm64: Destroy mpidr_data for 'late' vCPU creation
Date: Wed, 8 May 2024 06:50:57 +0000 [thread overview]
Message-ID: <Zjsg0d8Li1vIgyk2@linux.dev> (raw)
In-Reply-To: <87h6f8zu1z.wl-maz@kernel.org>
On Wed, May 08, 2024 at 07:39:20AM +0100, Marc Zyngier wrote:
> On Tue, 07 May 2024 20:29:12 +0100,
> Oliver Upton <oliver.upton@linux.dev> wrote:
> >
> > A particularly annoying userspace could create a vCPU after KVM has
> > computed mpidr_data for the VM, either by racing against VGIC
> > initialization or having a userspace irqchip.
> >
> > In any case, this means mpidr_data no longer fully describes the VM, and
> > attempts to find the new vCPU with kvm_mpidr_to_vcpu() will fail. The
> > fix is to discard mpidr_data altogether, as it is only a performance
> > optimization and not required for correctness. In all likelihood KVM
> > will recompute the mappings when KVM_RUN is called on the new vCPU.
> >
> > Note that reads of mpidr_data are not guarded by a lock; promote to RCU
> > to cope with the possibility of mpidr_data being invalidated at runtime.
> >
> > Fixes: 54a8006d0b49 ("KVM: arm64: Fast-track kvm_mpidr_to_vcpu() when mpidr_data is available")
> > Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> > ---
> > arch/arm64/kvm/arm.c | 49 ++++++++++++++++++++++++++++++++++++--------
> > 1 file changed, 40 insertions(+), 9 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index c4a0a35e02c7..0d845131a0e0 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -195,6 +195,22 @@ void kvm_arch_create_vm_debugfs(struct kvm *kvm)
> > kvm_sys_regs_create_debugfs(kvm);
> > }
> >
> > +static void kvm_destroy_mpidr_data(struct kvm *kvm)
> > +{
> > + struct kvm_mpidr_data *data;
> > +
> > + mutex_lock(&kvm->arch.config_lock);
> > +
> > + data = rcu_dereference_raw(kvm->arch.mpidr_data);
>
> I'm slightly worried by this. Why can't we use the "cooked" version?
> If anything I'd like to see a comment about this, as it is usually
> frowned upon.
No reason other than my own laziness... This really should be:
rcu_dereference_protected(kvm->arch.mpidr_data,
lockdep_is_held(&kvm->arch.config_lock));
since we're behind the update-side lock.
--
Thanks,
Oliver
prev parent reply other threads:[~2024-05-08 6:51 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-07 19:29 [PATCH] KVM: arm64: Destroy mpidr_data for 'late' vCPU creation Oliver Upton
2024-05-08 6:39 ` Marc Zyngier
2024-05-08 6:50 ` Oliver Upton [this message]
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