linux-kernel - Re: [PATCH v3] KVM: SEV: Mark nested locking of vcpu->lock

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Date:   Wed, 27 Apr 2022 18:04:23 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Gonda <pgonda@...gle.com>
Cc:     John Sperbeck <jsperbeck@...gle.com>,
        kvm list <kvm@...r.kernel.org>,
        David Rientjes <rientjes@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] KVM: SEV: Mark nested locking of vcpu->lock

On 4/26/22 21:06, Peter Gonda wrote:
> On Thu, Apr 21, 2022 at 9:56 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>>
>> On 4/20/22 22:14, Peter Gonda wrote:
>>>>>> svm_vm_migrate_from() uses sev_lock_vcpus_for_migration() to lock all
>>>>>> source and target vcpu->locks. Mark the nested subclasses to avoid false
>>>>>> positives from lockdep.
>>>> Nope. Good catch, I didn't realize there was a limit 8 subclasses:
>>> Does anyone have thoughts on how we can resolve this vCPU locking with
>>> the 8 subclass max?
>>
>> The documentation does not have anything.  Maybe you can call
>> mutex_release manually (and mutex_acquire before unlocking).
>>
>> Paolo
> 
> Hmm this seems to be working thanks Paolo. To lock I have been using:
> 
> ...
>                    if (mutex_lock_killable_nested(
>                                &vcpu->mutex, i * SEV_NR_MIGRATION_ROLES + role))
>                            goto out_unlock;
>                    mutex_release(&vcpu->mutex.dep_map, _THIS_IP_);
> ...
> 
> To unlock:
> ...
>                    mutex_acquire(&vcpu->mutex.dep_map, 0, 0, _THIS_IP_);
>                    mutex_unlock(&vcpu->mutex);
> ...
> 
> If I understand correctly we are fully disabling lockdep by doing
> this. If this is the case should I just remove all the '_nested' usage
> so switch to mutex_lock_killable() and remove the per vCPU subclass?

Yes, though you could also do:

	bool acquired = false;
	kvm_for_each_vcpu(...) {
		if (acquired)
			mutex_release(&vcpu->mutex.dep_map, _THIS_IP_);
		if (mutex_lock_killable_nested(&vcpu->mutex, role)
			goto out_unlock;
		acquired = true;
		...

and to unlock:

	bool acquired = true;
	kvm_for_each_vcpu(...) {
		if (!acquired)
			mutex_acquire(&vcpu->mutex.dep_map, 0, role, _THIS_IP_);
		mutex_unlock(&vcpu->mutex);
		acquired = false;
	}

where role is either 0 or SINGLE_DEPTH_NESTING and is passed to 
sev_{,un}lock_vcpus_for_migration.

That coalesces all the mutexes for a vm in a single subclass, essentially.

Paolo