Re: [PATCH v3 09/12] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use

[Paolo Bonzini <pbonzini@redhat.com>]

On 02/08/21 20:33, Maxim Levitsky wrote:
> +	if (!auto_eoi_old && auto_eoi_new) {
> +		if (atomic_inc_return(&hv->synic_auto_eoi_used) == 1)
> +			kvm_request_apicv_update(vcpu->kvm, false,
> +						APICV_INHIBIT_REASON_HYPERV);
> +	} else if (!auto_eoi_new && auto_eoi_old) {
> +		if (atomic_dec_return(&hv->synic_auto_eoi_used) == 0)
> +			kvm_request_apicv_update(vcpu->kvm, true,
> +						APICV_INHIBIT_REASON_HYPERV);

Hmm no, Reviewed-by rescinded.  This is racy, you cannot use atomics. 
The check for zero needs to happen within the lock.

The easiest way is to have a __kvm_request_apicv_update function that 
leaves it to the caller to take the lock.  Then synic_update_vector can do

	if (!!auto_eoi_old == !!auto_eoi_new)
		return;

	mutex_lock(&kvm->apicv_update_lock);
	bool was_active = hv->synic_auto_eoi_used;
	if (auto_eoi_new)
		hv->synic_auto_eoi_used++;
	else
		hv->synic_auto_eoi_used--;

	if (!!hv->synic_auto_eoi_used != !!was_active)
		__kvm_request_apicv_update(vcpu->kvm,
					   !!hv->synic_auto_eoi_used,
					   APICV_INHIBIT_REASON_HYPERV);
	mutex_unlock(&kvm->apicv_update_lock);

Please add a note to synic_auto_eoi_used that it is protected by 
apicv_update_lock.

Thanks,

Paolo