On Tue, Jan 21, 2025 at 04:09:24PM -0400, Jason Gunthorpe wrote: > On Tue, Jan 21, 2025 at 11:55:16AM -0800, Nicolin Chen wrote: > > > > > 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. > > > > > > > > So userspace should first read the header to decide whether or not > > > > to read a vEVENT. If header is overflows, it should skip the vEVENT > > > > struct and read the next header? > > > > > > Yes, but there won't be a next header. overflow would always be the > > > last thing in a read() response. If there is another event then > > > overflow is indicated by non-monotonic count. > > > > I am not 100% sure why "overflow would always be the last thing > > in a read() response". I thought that kernel should immediately > > report an overflow to user space when the vEVENTQ is overflowed. > > As below, if you observe overflow then it was at the end of the kernel > queue and there is no further events after it. So it should always end > up last. > > Perhaps we could enforce this directly in the kernel's read by making > it the only, first and last, response to read.
Hmm, since the overflow node is the last node in the list, isn't it already an enforcement it's the only/first/last node to read? (Assuming we choose to delete the overflow node from the list if new event can be inserted.) > > Yet, thinking about this once again: user space actually has its > > own queue. There's probably no point in letting it know about an > > overflow immediately when the kernel vEVENTQ overflows until its > > own user queue overflows after it reads the entire vEVENTQ so it > > can trigger a vHW event/irq to the VM? > > The kernel has no idea what userspace is doing, the kernel's job > should be to ensure timely delivery of all events, if an event is lost > it should ensure timely delivery of the lost event notification. There > is little else it can do. Yet, "timely" means still having an entire-queue-size-long delay since the overflow node is at the end of the queue, right? Thanks Nicolin
