linux-kernel - Re: [PATCH v6 0/6] iommufd: Add nesting infrastructure (part 2/2)

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Message-ID: <20231211130555.GB2944114@nvidia.com>
Date:   Mon, 11 Dec 2023 09:05:55 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Yi Liu <yi.l.liu@...el.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        "Giani, Dhaval" <Dhaval.Giani@....com>,
        Vasant Hegde <vasant.hegde@....com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        "joro@...tes.org" <joro@...tes.org>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.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>,
        "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>,
        "joao.m.martins@...cle.com" <joao.m.martins@...cle.com>,
        "Zeng, Xin" <xin.zeng@...el.com>,
        "Zhao, Yan Y" <yan.y.zhao@...el.com>
Subject: Re: [PATCH v6 0/6] iommufd: Add nesting infrastructure (part 2/2)

On Mon, Dec 11, 2023 at 08:36:46PM +0800, Yi Liu wrote:
> On 2023/12/11 10:29, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@...dia.com>
> > > Sent: Saturday, December 9, 2023 9:47 AM
> > > 
> > > What is in a Nested domain:
> > >   Intel: A single IO page table refereed to by a PASID entry
> > >          Each vDomain-ID,PASID allocates a unique nesting domain
> > >   AMD: A GCR3 table pointer
> > >        Nesting domains are created for every unique GCR3 pointer.
> > >        vDomain-ID can possibly refer to multiple Nesting domains :(
> > >   ARM: A CD table pointer
> > >        Nesting domains are created for every unique CD table top pointer.
> > 
> > this AMD/ARM difference is not very clear to me.
> > 
> > How could a vDomain-ID refer to multiple GCR3 pointers? Wouldn't it
> > lead to cache tag conflict when a same PASID entry in multiple GCR3 tables
> > points to different I/O page tables?
> 
> Perhaps due to only one DomainID in the DTE table indexed by BDF? Actually,
> the vDomainID will not be used to tag cache, the host DomainId would be
> used instead. @Jason?

The DomainID comes from the DTE table which is indexed by the RID, and
the DTE entry points to the GCR3 table. So the VM certainly can setup
a DTE table with multiple entires having the same vDomainID but
pointing to different GCR3's. So the VMM has to do *something* with
this.

Most likely this is not a useful thing to do. However what should the
VMM do when it sees this? Block a random DTE or push the duplication
down to real HW would be my options. I'd probably try to do the latter
just on the basis of better emulation.

Jason