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

[Nicolin Chen <nicolinc@nvidia.com>]

On Mon, Jan 13, 2025 at 03:54:33PM -0400, Jason Gunthorpe wrote:
> On Mon, Jan 13, 2025 at 11:47:52AM -0800, Nicolin Chen wrote:
> > > The other approach would be to add a sequence number to each event and
> > > let userspace detect the non-montonicity. It would require adding a
> > > header to the native ARM evt.
> > 
> > Yea, I thought about that. The tricky thing is that the header will
> > be a core-level header pairing with a driver-level vEVENTQ type and
> > can never change its length, though we can define a 64-bit flag that
> > can reserve the other 63 bits for future use?
> 
> The header format could be revised by changing the driver specific
> format tag.

Yea, we need a header format (or "header type") when the vEVENTQ
is allocated.

> You'd want to push a special event when the first overflow happens and
> probably also report a counter so userspace can know how many events
> got lost.

How about this:

enum iommufd_veventq_header_type {
	IOMMU_VEVENTQ_HEADER_TYPE_V1,
};

enum iommu_hwpt_pgfault_flags {
	IOMMU_VEVENT_HEADER_FLAGS_OVERFLOW = (1 << 0),
};

struct iommufd_vevent_header_v1 {
	__u64 flags;
	__u32 num_events;
	__u32 num_overflows; // valid if flag_overflow is set
};

> This seems most robust and simplest to implement..
> 
> I think I'd implement it by having a static overflow list entry so no
> memory allocation is needed and just keep moving that entry to the
> back of the list every time an event is lost. This way it will cover
> lost events due to memory outages too

Could double-adding the same static node to the list happen and
corrupt the list?

I think the vevent_header doesn't need to be exactly match with
the driver event. So, maybe a vEVENTQ object could hold a header
structure during iommu_veventq_alloc?

struct iommufd_veventq {
	...
	atomic_t num_events;
	atomic_t num_overflows;
	DECLARE_BITMAP(errors, 32);
	struct iommufd_vevent_header_v1 *header;
};

The header is a bit redundant to its upper three members though.

> For old formats like the fault queue you could return EOVERFLOW
> whenever the sequence number becomes discontiguous or it sees the
> overflow event..

So, IOMMU_OBJ_FAULT is safe to return EOVERFLOW via read(), as
you mentioned that it is self-limited right?

Thanks
Nicolin