Re: [PATCH 2/3] KVM: x86: Remove use of apicv_update_lock when toggling guest debug state

Paolo Bonzini
2025-02-04T17:58:44Z

On 2/4/25 18:51, Sean Christopherson wrote:
> On Tue, Feb 04, 2025, Naveen N Rao wrote:
>> On Mon, Feb 03, 2025 at 09:00:05PM -0500, Maxim Levitsky wrote:
>>> On Mon, 2025-02-03 at 22:33 +0530, Naveen N Rao (AMD) wrote:
>>>> apicv_update_lock is not required when querying the state of guest
>>>> debug in all the vcpus. Remove usage of the same, and switch to
>>>> kvm_set_or_clear_apicv_inhibit() helper to simplify the code.
>>>
>>> It might be worth to mention that the reason why the lock is not needed,
>>> is because kvm_vcpu_ioctl from which this function is called takes 'vcpu->mutex'
>>> and thus concurrent execution of this function is not really possible.
>>
>> Looking at this again, that looks to be a vcpu-specific lock, so I guess
>> it is possible for multiple vcpus to run this concurrently?
>
> Correct.

And this patch is incorrect. Because there is a store and many loads,
you have the typical race when two vCPUs set blockirq at the same time

vcpu 0 vcpu 1
--------------- --------------
set vcpu0->guest_debug
clear vcpu1->guest_debug
read vcpu0->guest_debug
read vcpu1->guest_debug
set inhibit
read stale vcpu0->guest_debug
read vcpu1->guest_debug
clear inhibit

But since this is really a slow path, why even bother optimizing it?

Paolo