Re: [PATCH v5 08/14] iommufd/viommu: Add iommufd_viommu_report_event helper

[Nicolin Chen <nicolinc@nvidia.com>]

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