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

[Paolo Bonzini]

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