linux-kernel - Re: [PATCH v2 2/3] iommu/sva: Stop using ioasid

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Message-ID: <Y+0okzkbTh6mw0EM@nvidia.com>
Date:   Wed, 15 Feb 2023 14:46:43 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Robin Murphy <robin.murphy@....com>,
        Will Deacon <will@...nel.org>,
        David Woodhouse <dwmw2@...radead.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>
Subject: Re: [PATCH v2 2/3] iommu/sva: Stop using ioasid_set for SVA

On Wed, Feb 15, 2023 at 10:38:22AM -0800, Jacob Pan wrote:
> Hi Kevin,
> 
> On Wed, 15 Feb 2023 03:24:18 +0000, "Tian, Kevin" <kevin.tian@...el.com>
> wrote:
> 
> > > From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> > > Sent: Tuesday, February 14, 2023 2:44 PM
> > > 
> > > From: Jason Gunthorpe <jgg@...dia.com>
> > > 
> > > Instead SVA drivers can use a simple global IDA to allocate PASIDs for
> > > each mm_struct.
> > > 
> > > Future work would be to allow drivers using the SVA APIs to reserve
> > > global PASIDs from this IDA for their internal use, eg with the DMA API
> > > PASID support.  
> > 
> > I think DMA API PASID will need new API around this IDA. SVA APIs should
> > be just for SVA.
> > 
> > > +void mm_pasid_drop(struct mm_struct *mm)
> > >  {
> > > -	return mmget_not_zero(mm);
> > > -}
> > > +	pr_alert("%s %d", __func__, mm->pasid);
> > > +	if (likely(!pasid_valid(mm->pasid)))
> > > +		return;
> > > 
> > > -/**
> > > - * iommu_sva_find() - Find mm associated to the given PASID
> > > - * @pasid: Process Address Space ID assigned to the mm
> > > - *
> > > - * On success a reference to the mm is taken, and must be released with
> > > mmput().
> > > - *
> > > - * Returns the mm corresponding to this PASID, or an error if not
> > > found.
> > > - */
> > > -struct mm_struct *iommu_sva_find(ioasid_t pasid)
> > > -{
> > > -	return ioasid_find(&iommu_sva_pasid, pasid, __mmget_not_zero);
> > > +	ida_free(&iommu_global_pasid_ida, mm->pasid);
> > >  }  
> > 
> > when moving this function following line is missed:
> > 
> > 	mm->pasid = INVALID_IOASID;
> > 
> not needed, mm is gone by now.
> 
> > btw the current placement looks asymmetric. We now have
> > mm_pasid_init() and mm_pasid_set() in mm.h while mm_pasid_drop()
> > in iommu-sva.c.
> > Is it cleaner to introduce a iommu_sva_free_pasid() helper and then
> > call it from mm_pasid_drop() in mm.h?
> That is what we had before, it seems logical to me. Jason?

I don't much like a maze of functions calling each other.

Renaming the helper might help, especially if it comes from iommu.h

Jason