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

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Date:   Tue, 21 Apr 2020 14:51:14 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Jean-Philippe Brucker <jean-philippe@...aro.org>
Cc:     Joerg Roedel <joro@...tes.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Yi Liu <yi.l.liu@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Jonathan Cameron <jic23@...nel.org>,
        Eric Auger <eric.auger@...hat.com>,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH 03/10] iommu/ioasid: Introduce per set allocation APIs

Hi Jean,

Sorry for the late reply, been trying to redesign the notification part.

On Tue, 7 Apr 2020 13:01:07 +0200
Jean-Philippe Brucker <jean-philippe@...aro.org> wrote:

> 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.
> 
much better.

> > 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.
> 
for vt-d, the per vm token (preferrably mm) will be used by kvm to
manage its PASID translation table.
when kvm receives a notification about a new guest-host PASID mapping,
it needs to know which vm it belongs to. So if mm is used as token,
both vfio and kvm can identify PASID ownership.

> > 
> > /**
> >  * 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.
> 
Sounds good. still pass a token. Thanks for the idea.

> Thanks,
> Jean

[Jacob Pan]