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

Naveen N Rao
2025-02-05T11:23:24Z

On Tue, Feb 04, 2025 at 09:51:22AM -0800, 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.
>
> > In reality, this looks to be coming in from a vcpu ioctl from userspace,
> > so this is probably not being invoked concurrently today.
> >
> > Regardless, I wonder if moving this to a per-vcpu inhibit might be a
> > better way to address this.
>
> No, this is a slow path.

My comment was more from the point of view of correctness, rather than
performance (with the goal of removing use of apicv_update_lock) --
similar to the issue with IRQWIN needing to maintain per-vcpu state. My
naive understanding of Maxim's mail was that we would introduce per-vcpu
inhibit field to maintain per-vcpu inhibit state, but not actually
inhibit AVIC on a per-vcpu basis :)

Thanks,
Naveen