On Fri, Jan 17, 2025 at 02:11:15PM -0800, Nicolin Chen wrote: > +/** > + * struct iommufd_vevent_header - Virtual Event Header for a vEVENTQ Status > + * @state: One of enum iommu_veventq_state I'd probably just make this a flag with overflow as the only current flag? > + * @counter: A counter reflecting the state of the vEVENTQ > + * ---------------------------------------------------------------------------- > + * | @state | @counter | > + * ---------------------------------------------------------------------------- > + * | IOMMU_VEVENTQ_STATE_OK | number of readable vEVENTs in the vEVENTQ | > + * | IOMMU_VEVENTQ_STATE_OVERFLOW | number of missed vEVENTs since overflow | > + * ---------------------------------------------------------------------------- When I said counter I literally ment a counter of the number of events that were sent into the queue. So if events are dropped there is a trivial gap in the count. Very easy to implement IOMMU_VEVENTQ_STATE_OVERFLOW with a 0 length event is seen if events have been lost and no subsequent events are present. It exists to ensure timely delivery of the overflow event to userspace. counter will be the sequence number of the next successful event. If events are lost in the middle of the queue then flags will remain 0 but counter will become non-montonic. A counter delta > 1 indicates that many events have been lost. Jason
