Re: [PATCH v6 05/14] iommufd: Add IOMMUFD_OBJ_VEVENTQ and IOMMUFD_CMD_VEVENTQ_ALLOC

[Nicolin Chen <nicolinc@nvidia.com>]

On Tue, Feb 18, 2025 at 11:29:59AM -0400, Jason Gunthorpe wrote:
> On Fri, Jan 24, 2025 at 04:30:34PM -0800, Nicolin Chen wrote:
> > +	list_add_tail(&vevent->node, &eventq->deliver);
> > +	vevent->on_list = true;
> > +	vevent->header.sequence = atomic_read(&veventq->sequence);
> > +	if (atomic_read(&veventq->sequence) == INT_MAX)
> > +		atomic_set(&veventq->sequence, 0);
> > +	else
> > +		atomic_inc(&veventq->sequence);
> > +	spin_unlock(&eventq->lock);
> 
> This is all locked, we don't need veventq->sequence to be an atomic?
> 
> The bounding can be done with some simple math:
> 
>   veventq->sequence = (veventq->sequence + 1) & INT_MAX;

Ack. Perhaps we can reuse eventq->lock to fence @num_events too.

> > +static struct iommufd_vevent *
> > +iommufd_veventq_deliver_fetch(struct iommufd_veventq *veventq)
> > +{
> > +	struct iommufd_eventq *eventq = &veventq->common;
> > +	struct list_head *list = &eventq->deliver;
> > +	struct iommufd_vevent *vevent = NULL;
> > +
> > +	spin_lock(&eventq->lock);
> > +	if (!list_empty(list)) {
> > +		vevent = list_first_entry(list, struct iommufd_vevent, node);
> > +		list_del(&vevent->node);
> > +		vevent->on_list = false;
> > +	}
> > +	/* Make a copy of the overflow node for copy_to_user */
> > +	if (vevent == &veventq->overflow) {
> > +		vevent = kzalloc(sizeof(*vevent), GFP_ATOMIC);
> > +		if (vevent)
> > +			memcpy(vevent, &veventq->overflow, sizeof(*vevent));
> > +	}
> 
> This error handling is wonky, if we can't allocate then we shouldn't
> have done the list_del. Just return NULL which will cause
> iommufd_veventq_fops_read() to exist and userspace will try again.

OK.

We have two cases to support here:
1) Normal vevent node -- list_del and return the node.
2) Overflow node -- list_del and return a copy.

I think we can do:
	if (!list_empty(list)) {
		struct iommufd_vevent *next;

		next = list_first_entry(list, struct iommufd_vevent, node);
		if (next == &veventq->overflow) {
			/* Make a copy of the overflow node for copy_to_user */
			vevent = kzalloc(sizeof(*vevent), GFP_ATOMIC);
			if (!vevent)
				goto out_unlock;
		}
		list_del(&next->node);
		if (vevent)
			memcpy(vevent, next, sizeof(*vevent));
		else
			vevent = next;
	}

> > @@ -403,6 +531,10 @@ static int iommufd_eventq_fops_release(struct inode *inode, struct file *filep)
> >  {
> >  	struct iommufd_eventq *eventq = filep->private_data;
> >  
> > +	if (eventq->obj.type == IOMMUFD_OBJ_VEVENTQ) {
> > +		atomic_set(&eventq_to_veventq(eventq)->sequence, 0);
> > +		atomic_set(&eventq_to_veventq(eventq)->num_events, 0);
> > +	}
> 
> Why? We are about to free the memory?

Ack. I thought about a re-entry of an open(). But release() does
lose the event_fd completely, and user space wouldn't be able to
open the same fd again.

> > +int iommufd_veventq_alloc(struct iommufd_ucmd *ucmd)
> > +{
> > +	struct iommu_veventq_alloc *cmd = ucmd->cmd;
> > +	struct iommufd_veventq *veventq;
> > +	struct iommufd_viommu *viommu;
> > +	int fdno;
> > +	int rc;
> > +
> > +	if (cmd->flags || cmd->type == IOMMU_VEVENTQ_TYPE_DEFAULT)
> > +		return -EOPNOTSUPP;
> > +	if (!cmd->veventq_depth)
> > +		return -EINVAL;
> 
> Check __reserved for 0 too

Kevin is suggesting a 32-bit flag field, so I think we can drop
the __reserved in that case.

Thanks
Nicolin