Re: [PATCH 03/10] iommu/ioasid: Introduce per set allocation APIs

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

On Mon, Apr 06, 2020 at 01:02:45PM -0700, Jacob Pan wrote:
> > > +	sdata = kzalloc(sizeof(*sdata), GFP_KERNEL);
> > > +	if (!sdata)
> > > +		return -ENOMEM;  
> > 
> > I don't understand why we need this structure at all, nor why we need
> > the SID. Users have already allocated an ioasid_set, so why not just
> > stick the content of ioasid_set_data in there, and pass the
> > ioasid_set pointer to ioasid_alloc()?
> > 
> 
> My thinking was that ioasid_set is an opaque user token, e.g. we use mm
> to identify a common set belong to a VM.
> 
> This sdata is an IOASID internal structure for managing & servicing per
> set data. If we let user fill in the content, some of the entries need
> to be managed by the IOASID code under a lock.

We don't have to let users fill the content. A bit like iommu_domain:
device drivers don't modify it, they pass it to iommu_map() rather than
passing a domain ID.

> IMO, not suitable to let user allocate and manage.
> 
> Perhaps we should rename struct ioasid_set to ioasid_set_token?

Is the token actually used anywhere?  As far as I can tell VFIO does its
own uniqueness check before calling ioasid_alloc_set(), and consumers of
notifications don't read the token.

> 
> /**
>  * struct ioasid_set_data - Meta data about ioasid_set
>  *
>  * @token:	Unique to identify an IOASID set
>  * @xa:		XArray to store ioasid_set private ID to
> system-wide IOASID
>  *		mapping
>  * @max_id:	Max number of IOASIDs can be allocated within the set
>  * @nr_id	Number of IOASIDs allocated in the set
>  * @sid		ID of the set
>  */
> struct ioasid_set_data {
> 	struct ioasid_set *token;
> 	struct xarray xa;
> 	int size;
> 	int nr_ioasids;
> 	int sid;
> 	struct rcu_head rcu;
> };

How about we remove the current ioasid_set, call this structure ioasid_set
instead of ioasid_set_data, and have ioasid_alloc_set() return it, rather
than requiring users to allocate the ioasid_set themselves?

	struct ioasid_set *ioasid_alloc_set(ioasid_t quota):

This way ioasid_set is opaque to users (we could have the definition in
ioasid.c), but it can be passed to ioasid_alloc() and avoids the lookup by
SID. Could also add the unique token as a void * argument to
ioasid_alloc_set(), if needed.

Thanks,
Jean