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

Naveen N Rao
2025-02-04T14:25:50Z

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?

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.

Thanks,
Naveen