linux-kernel - Re: [PATCH v7 06/10] iommu/sva: Refactoring iommu_sva_bind/unbind

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Date:   Wed, 25 May 2022 08:29:49 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Raj, Ashok" <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        Vinod Koul <vkoul@...nel.org>,
        Eric Auger <eric.auger@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 06/10] iommu/sva: Refactoring
 iommu_sva_bind/unbind_device()

On Wed, May 25, 2022 at 02:04:49AM +0000, Tian, Kevin wrote:
> > From: Jean-Philippe Brucker <jean-philippe@...aro.org>
> > Sent: Tuesday, May 24, 2022 6:58 PM
> > 
> > On Tue, May 24, 2022 at 10:22:28AM +0000, Tian, Kevin wrote:
> > > > From: Lu Baolu <baolu.lu@...ux.intel.com>
> > > > Sent: Thursday, May 19, 2022 3:21 PM
> > > >
> > > > The existing iommu SVA interfaces are implemented by calling the SVA
> > > > specific iommu ops provided by the IOMMU drivers. There's no need for
> > > > any SVA specific ops in iommu_ops vector anymore as we can achieve
> > > > this through the generic attach/detach_dev_pasid domain ops.
> > >
> > > set/block_pasid_dev, to be consistent.
> > >
> > > > +
> > > > +	mutex_lock(&iommu_sva_lock);
> > > > +	/* Search for an existing domain. */
> > > > +	domain = iommu_get_domain_for_dev_pasid(dev, mm->pasid);
> > > > +	if (domain) {
> > > > +		sva_domain = to_sva_domain(domain);
> > > > +		refcount_inc(&sva_domain->bond.users);
> > > > +		goto out_success;
> > > > +	}
> > > > +
> > >
> > > why would one device/pasid be bound to a mm more than once?
> > 
> > Device drivers can call bind() multiple times for the same device and mm,
> > for example if one process wants to open multiple accelerator queues.
> > 
> 
> Is it clearer to have a sva_bond_get/put() pair instead of calling
> bind() multiple times here? 

I don't think it's clearer, and it would force device drivers to keep
track of {dev, mm} pairs, when the IOMMU subsystem already does that.
At the moment a device driver calls

	bond = iommu_sva_bind_device(dev, mm)

for each ADI that it wants to assign to userspace. If a process happens to
want multiple ADIs on one device, then the {dev, mm} parameters are the
same and bind() returns the same bond. Since the IOMMU driver needs to
track these anyway, it might as well refcount them.

Thanks,
Jean