linux-kernel - RE: [RFC 3/8] iommufd: Support attach/replace hwpt per pasid

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Date:   Thu, 28 Sep 2023 01:54:36 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Baolu Lu <baolu.lu@...ux.intel.com>
CC:     "Liu, Yi L" <yi.l.liu@...el.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "nicolinc@...dia.com" <nicolinc@...dia.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "mjrosato@...ux.ibm.com" <mjrosato@...ux.ibm.com>,
        "chao.p.peng@...ux.intel.com" <chao.p.peng@...ux.intel.com>,
        "yi.y.sun@...ux.intel.com" <yi.y.sun@...ux.intel.com>,
        "peterx@...hat.com" <peterx@...hat.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "shameerali.kolothum.thodi@...wei.com" 
        <shameerali.kolothum.thodi@...wei.com>,
        "lulu@...hat.com" <lulu@...hat.com>,
        "suravee.suthikulpanit@....com" <suravee.suthikulpanit@....com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "Duan, Zhenzhong" <zhenzhong.duan@...el.com>,
        "Martins, Joao" <joao.m.martins@...cle.com>
Subject: RE: [RFC 3/8] iommufd: Support attach/replace hwpt per pasid

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Wednesday, September 27, 2023 11:44 PM
> 
> On Wed, Sep 27, 2023 at 10:49:29AM +0800, Baolu Lu wrote:
> > On 9/26/23 5:26 PM, Yi Liu wrote:
> > > From: Kevin Tian<kevin.tian@...el.com>
> > >
> > > This introduces three APIs for device drivers to manage pasid attach/
> > > replace/detach.
> > >
> > >      int iommufd_device_pasid_attach(struct iommufd_device *idev,
> > > 				    u32 pasid, u32 *pt_id);
> > >      int iommufd_device_pasid_replace(struct iommufd_device *idev,
> > > 				     u32 pasid, u32 *pt_id);
> > >      void iommufd_device_pasid_detach(struct iommufd_device *idev,
> > > 				     u32 pasid);
> >
> > I am a bit puzzled. Do we really need both attach and replace interfaces
> > to install a hwpt onto a pasid on device? The IOMMUFD already tracks the
> > connections between hwpt and {device, pasid}, so it could easily call
> > the right iommu interfaces (attach vs. replace). Perhaps I overlooked
> > previous discussion on this.
> 
> It was a decision that attach will fail if something is already
> attached..
> 
> But for this API we could go the way of the iommu code and have only
> 'set' and 'unset' as the two operations.
> 

I'm not sure the benefit of doing so. Instead it makes the caller side
vfio more confusing by using attach/replace/detach for device vs.
using set/unset for pasid?