Re: [PATCH 1/2] KVM: x86: Fix deadlock in kvm_vm_ioctl_set_msr_filter()

[Sean Christopherson <seanjc@google.com>]

On Fri, Jan 06, 2023, Paolo Bonzini wrote:
> On 1/5/23 23:23, Sean Christopherson wrote:
> > Ha!  Case in point.  The aforementioned Xen code blatantly violates KVM's locking
> > rules:
> > 
> >    - kvm->lock is taken outside vcpu->mutex
> 
> Ouch yeah, that's not salvageable.  Anything that takes kvm->lock inside
> kvm->srcu transitively has to be taking kvm->lock inside vcpu->mutex as
> well.
> 
> In abstract I don't think that "vcpu->mutex inside kvm->lock" would be a
> particularly problematic rule; kvm->lock critical sections are much shorter
> than vcpu->mutex which covers all of KVM_RUN for example, and that hints at
> making vcpu->mutex the *outer* mutex.  However, I completely forgot the
> sev_lock_vcpus_for_migration case, which is the exception that... well,
> disproves the rule.

Ya, and there are plenty more instances outside of x86.

ARM's vGIC stuff also does similar things, see lock_all_vcpus().

PPC's kvmppc_xive_release() and kvmppc_xics_release().

s390's kvm_s390_cpus_from_pv().