Re: [RFC PATCH v3 5/6] KVM: SVM: Add support for AMD IOMMU Guest APIC Physical Processor Interrupt (GAPPI)

[Sairaj Kodilkar <sarunkod@amd.com>]

On 8/11/2026 6:10 AM, Sean Christopherson wrote:
> On Mon, Jul 13, 2026, Sairaj Kodilkar wrote:
>> On 7/13/2026 4:41 PM, sashiko-bot@kernel.org wrote:
>>> [Severity: High]
>>> Does this lock ordering create an ABBA deadlock?
>>>
>>> When a vCPU is preempted, the scheduler executes kvm_sched_out() while holding
>>> the runqueue lock (rq_lock). KVM then calls __avic_vcpu_put(), which in turn
>>> calls avic_add_vcpu_to_gappi_wakeup_list(). This acquires the
>>> gappi_vcpu_wakeup_list_lock, establishing an ordering of rq_lock ->
>>> gappi_vcpu_wakeup_list_lock.
>>>
>>> Conversely, when a GAPPI interrupt arrives, avic_gappi_wakeup_handler()
>>> acquires gappi_vcpu_wakeup_list_lock and calls kvm_vcpu_wake_up(). The wakeup
>>> path then attempts to acquire the target task's runqueue lock, establishing
>>> the reverse order: gappi_vcpu_wakeup_list_lock -> rq_lock.
>>>
>>> This inversion will likely trigger a lockdep ABBA deadlock splat and violates
>>> the scheduler locking hierarchy.
>>
>> For this condition to exist, Following two conditions must be true.
>> 1. avic_vcpu_put() and gappi interrupt handler() must be on same
>> physical CPU.
>> 2. GAPPI must preempt the __avic_vcpu_put() once it has acquired
>> gappi_vcpu_wakeup_list_lock().
>>
>> The second condition is not possible since __avic_vcpu_put() disables
>> the interrupt while aquiring the ir_list lock.
>>
>> Hence ABBA deadlock is not possible
> 
> I wouldn't be surprised if you need to tell lockdep that, e.g. see commit
> c0b8dcabb2cd ("KVM: VMX: Use separate subclasses for PI wakeup lock to squash
> false positive").

Will do this in next version.

Thanks
Sairaj