Subject: Re: [PATCH kernel 3/3] powerpc/iommu: Add iommu_ops to report capabilities and allow blocking domains

From: Jason Gunthorpe <[email-masked]>
Date: Mon, 18 Jul 2022 15:09:24 -0300

On Thu, Jul 14, 2022 at 06:18:22PM +1000, Alexey Kardashevskiy wrote:

> +/*
> + * A simple iommu_ops to allow less cruft in generic VFIO code.
> + */
> +static bool spapr_tce_iommu_capable(enum iommu_cap cap)
> +{
> +	switch (cap) {
> +	case IOMMU_CAP_CACHE_COHERENCY:

I would add a remark here that it is because vfio is going to use
SPAPR mode but still checks that the iommu driver support coherency -
with out that detail it looks very strange to have caps without
implementing unmanaged domains

> +static struct iommu_domain *spapr_tce_iommu_domain_alloc(unsigned int type)
> +{
> +	struct iommu_domain *dom;
> +
> +	if (type != IOMMU_DOMAIN_BLOCKED)
> +		return NULL;
> +
> +	dom = kzalloc(sizeof(*dom), GFP_KERNEL);
> +	if (!dom)
> +		return NULL;
> +
> +	dom->geometry.aperture_start = 0;
> +	dom->geometry.aperture_end = ~0ULL;
> +	dom->geometry.force_aperture = true;

A blocked domain doesn't really have an aperture, all DMA is rejected,
so I think these can just be deleted and left at zero.

Generally I'm suggesting drivers just use a static singleton instance
for the blocked domain instead of the allocation like this, but that
is a very minor nit.

> +static struct iommu_device *spapr_tce_iommu_probe_device(struct device *dev)
> +{
> +	struct pci_dev *pdev;
> +	struct pci_controller *hose;
> +
> +	/* Weirdly iommu_device_register() assigns the same ops to all buses */
> +	if (!dev_is_pci(dev))
> +		return ERR_PTR(-EPERM);

Less "weirdly", more by design. The iommu driver should check if the
given struct device is supported or not, it isn't really a bus
specific operation.

> +static struct iommu_group *spapr_tce_iommu_device_group(struct device *dev)
> +{
> +	struct pci_controller *hose;
> +	struct pci_dev *pdev;
> +
> +	/* Weirdly iommu_device_register() assigns the same ops to all buses */
> +	if (!dev_is_pci(dev))
> +		return ERR_PTR(-EPERM);

This doesn't need repeating, if probe_device() fails then this will
never be called.

> +static int spapr_tce_iommu_attach_dev(struct iommu_domain *dom,
> +				      struct device *dev)
> +{
> +	struct iommu_group *grp = iommu_group_get(dev);
> +	struct iommu_table_group *table_group;
> +	int ret = -EINVAL;
> +
> +	if (!grp)
> +		return -ENODEV;
> +
> +	table_group = iommu_group_get_iommudata(grp);
> +
> +	if (dom->type == IOMMU_DOMAIN_BLOCKED)
> +		ret = table_group->ops->take_ownership(table_group);

Ideally there shouldn't be dom->type checks like this.

The blocking domain should have its own iommu_domain_ops that only
process the blocking operation. Ie call this like
spapr_tce_iommu_blocking_attach_dev()

Instead of having a "default_domain_ops" leave it NULL and create a
spapr_tce_blocking_domain_ops with these two functions and assign it
to domain->ops when creating. Then it is really clear these functions
are only called for the DOMAIN_BLOCKED type and you don't need to
check it.

> +static void spapr_tce_iommu_detach_dev(struct iommu_domain *dom,
> +				       struct device *dev)
> +{
> +	struct iommu_group *grp = iommu_group_get(dev);
> +	struct iommu_table_group *table_group;
> +
> +	table_group = iommu_group_get_iommudata(grp);
> +	WARN_ON(dom->type != IOMMU_DOMAIN_BLOCKED);
> +	table_group->ops->release_ownership(table_group);
> +}

Ditto

> +struct iommu_group *pSeries_pci_device_group(struct pci_controller *hose,
> +					     struct pci_dev *pdev)
> +{
> +	struct device_node *pdn, *dn = pdev->dev.of_node;
> +	struct iommu_group *grp;
> +	struct pci_dn *pci;
> +
> +	pdn = pci_dma_find(dn, NULL);
> +	if (!pdn || !PCI_DN(pdn))
> +		return ERR_PTR(-ENODEV);
> +
> +	pci = PCI_DN(pdn);
> +	if (!pci->table_group)
> +		return ERR_PTR(-ENODEV);
> +
> +	grp = pci->table_group->group;
> +	if (!grp)
> +		return ERR_PTR(-ENODEV);
> +
> +	return iommu_group_ref_get(grp);

Not for this series, but this is kind of backwards, the driver
specific data (ie the table_group) should be in
iommu_group_get_iommudata()...

> diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
> index 8a65ea61744c..3b53b466e49b 100644
> --- a/drivers/vfio/vfio_iommu_spapr_tce.c
> +++ b/drivers/vfio/vfio_iommu_spapr_tce.c
> @@ -1152,8 +1152,6 @@ static void tce_iommu_release_ownership(struct tce_container *container,
>  	for (i = 0; i < IOMMU_TABLE_GROUP_MAX_TABLES; ++i)
>  		if (container->tables[i])
>  			table_group->ops->unset_window(table_group, i);
> -
> -	table_group->ops->release_ownership(table_group);
>  }
>  
>  static long tce_iommu_take_ownership(struct tce_container *container,
> @@ -1161,10 +1159,6 @@ static long tce_iommu_take_ownership(struct tce_container *container,
>  {
>  	long i, ret = 0;
>  
> -	ret = table_group->ops->take_ownership(table_group);
> -	if (ret)
> -		return ret;
> -
>  	/* Set all windows to the new group */
>  	for (i = 0; i < IOMMU_TABLE_GROUP_MAX_TABLES; ++i) {
>  		struct iommu_table *tbl = container->tables[i];
> @@ -1183,8 +1177,6 @@ static long tce_iommu_take_ownership(struct tce_container *container,
>  	for (i = 0; i < IOMMU_TABLE_GROUP_MAX_TABLES; ++i)
>  		table_group->ops->unset_window(table_group, i);
>  
> -	table_group->ops->release_ownership(table_group);
> -

This is great, makes alot of sense.

Anyhow, it all looks fine to me as is even:

Reviewed-by: Jason Gunthorpe <[email-masked]>

Jason