Re: [RFC] iommufd: Use accurate dev_id in the PRI forwarding path

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Mar 06, 2025 at 03:09:34PM +0800, Yi Liu wrote:
> +bool iommu_group_is_singular(struct iommu_group *group)
> +{
> +	bool res;
> +
> +	mutex_lock(&group->mutex);
> +	res = list_count_nodes(&group->devices) == 1;
> +	mutex_unlock(&group->mutex);
> +
> +	return res;
> +}
> +EXPORT_SYMBOL_NS_GPL(iommu_group_is_singular, "IOMMUFD_INTERNAL");

I'm not so excited about something racy like this..

I still think this is a bit wrong, we only care about aliases, not
multi device groups.

Aliases are created here:

	if (pci_for_each_dma_alias(pdev, get_pci_alias_or_group, &data))
		return data.group;

And maybe in a few other places..

Can you tag the group that it has aliases in it directly during group
creation so there are no races?

I've long had a desire to more clearly separate the two cases of
groups:

 1) The HW cannot tell the different struct device sources on a per
    DMA basis
 2) There is a security intertwinement between the devices but they
    can all run separately in the IOMMU HW

Maybe it is not so hard?

Jason