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

[Maxim Levitsky <mlevitsk@redhat.com>]

On Tue, 2025-02-04 at 18:58 +0100, Paolo Bonzini wrote:
> 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
> 


Paolo, you are absolutely right! the vcpu mutex only prevents concurrent ioctl
on a same vcpu, but not on different vcpus, and without locking of course
this patch isn't going to work. The per-vcpu mutex is not something I know well,
and I only recently made aware of it, so I mixed this thing up.

So yes, some kind of lock is needed here.

Best regards,
     Maxim Levitsky