RE: [PATCH v2 1/3] iommu: Make single-device group for PASID explicit

[Tian, Kevin]

> From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> Sent: Monday, August 14, 2023 9:18 AM
> 
> The PASID interfaces have always supported only single-device groups.
> This was first introduced in commit 26b25a2b98e45 ("iommu: Bind process
> address spaces to devices"), and has been kept consistent in subsequent
> commits.
> 
> However, the core code doesn't explicitly check for this requirement
> after commit 201007ef707a8 ("PCI: Enable PASID only when ACS RR & UF
> enabled on upstream path"), which made this requirement implicit.
> 
> Restore the check to make it explicit that the PASID interfaces only
> support devices belonging to single-device groups.
> 
> Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> ---
>  drivers/iommu/iommu.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 71b9c41f2a9e..f1eba60e573f 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -3408,6 +3408,11 @@ int iommu_attach_device_pasid(struct
> iommu_domain *domain,
>  		return -ENODEV;
> 
>  	mutex_lock(&group->mutex);
> +	if (list_count_nodes(&group->devices) != 1) {
> +		ret = -EINVAL;
> +		goto out_unlock;
> +	}
> +

I wonder whether we should also block adding new device to this
group once the single-device has pasid enabled. Otherwise the
check alone at attach time doesn't mean this group won't be
expanded to have multiple devices later.