linux-kernel - Re: [PATCH v10 08/12] iommu/sva: Refactoring iommu_sva_bind/unbind

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Date:   Mon, 25 Jul 2022 11:47:58 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Joerg Roedel <joro@...tes.org>,
        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>,
        Zhangfei Gao <zhangfei.gao@...aro.org>,
        "Zhu, Tony" <tony.zhu@...el.com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v10 08/12] iommu/sva: Refactoring
 iommu_sva_bind/unbind_device()

On Mon, Jul 25, 2022 at 06:22:06PM +0800, Baolu Lu wrote:
> On 2022/7/25 15:50, Tian, Kevin wrote:
> > > From: Baolu Lu <baolu.lu@...ux.intel.com>
> > > Sent: Sunday, July 24, 2022 9:48 PM
> > > > 
> > > > The API is really refcounting the PASID:
> > > > 
> > > > > +struct iommu_sva *iommu_sva_bind_device(struct device *dev,
> > > > > +					struct mm_struct *mm);
> > > > > +void iommu_sva_unbind_device(struct iommu_sva *handle);
> > > > 
> > > > So what you need to do is store that 'iommu_sva' in the group's PASID
> > > > xarray.
> > > > 
> > > > The bind logic would be
> > > > 
> > > >     sva = xa_load(group->pasid, mm->pasid)
> > > >     if (sva)
> > > >        refcount_inc(sva->users)
> > > >        return sva
> > > >     sva = kalloc
> > > >     sva->domain = domain
> > > >     xa_store(group->pasid, sva);
> > > 
> > > Thanks for the suggestion. It makes a lot of sense to me.
> > > 
> > > Furthermore, I'd like to separate the generic data from the caller-
> > > specific things because the group->pasid_array should also be able to
> > > serve other usages. Hence, the attach/detach_device_pasid interfaces
> > > might be changed like below:
> > > 
> > > /* Collection of per-pasid IOMMU data */
> > > struct group_pasid {
> > > 	struct iommu_domain *domain;
> > > 	void *priv;
> > > };
> > > 
> > 
> > Is there any reason why pasid refcnt is sva specific and needs to be
> > in a priv field?
> 
> I am going to store the iommu_sva data which represents the bind
> relationship between device and domain.

Why do you need that?

If you are starting at the pasid xarray then you already know the
group/device, so we don't need to store it again.

The only thing needed is the refcount so just store a refcount in this
structure and be done with it. If someone needs to add something later
then we can use a union or something, but right now adding an untagged
void * is bad.

Jason