linux-kernel - Re: [PATCH v2 00/11] iommufd: Add nesting infrastructure

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Message-ID: <ZJLnTt/lCyTYBm7G@nvidia.com>
Date:   Wed, 21 Jun 2023 09:04:30 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.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>,
        "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>,
        "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>
Subject: Re: [PATCH v2 00/11] iommufd: Add nesting infrastructure

On Wed, Jun 21, 2023 at 06:02:21AM +0000, Tian, Kevin wrote:
> > > My understanding of ARM SMMU is that from host p.o.v. the CD is the
> > > S1 in the nested configuration. 'identity' is one configuration in the CD
> > > then it's in the business of nesting.
> > 
> > I think it is the same. A CD doesn't come into the picture until the
> > guest installs a CD pointing STE. Until that time the S2 is being used
> > as identity.
> > 
> > It sounds like the same basic flow.
> 
> After a CD table is installed in a STE I assume the SMMU still allows to
> configure an individual CD entry as identity? e.g. while vSVA is enabled
> on a device the guest can continue to keep CD#0 as identity when the
> default domain of the device is set as 'passthrough'. In this case the
> IOAS still needs to gain reserved regions even though S2 is not directly
> attached from host p.o.v.

In any nesting configuration the hypervisor cannot directly restrict
what IOVA the guest will use. The VM could make a normal nest and try
to use unusable IOVA. Identity is not really special.

The VMM should construct the guest memory map so that an identity
iommu_domain can meet the reserved requirements - it needs to do this
anyhow for the initial boot part. It shouuld try to forward the
reserved regions to the guest via ACPI/etc.

Being able to explicitly load reserved regions into an IOAS seems like
a useful way to help construct this.

Jason