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

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Message-ID: <ZJwpQV6oBBsIAtHD@nvidia.com>
Date:   Wed, 28 Jun 2023 09:36:17 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Nicolin Chen <nicolinc@...dia.com>,
        "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>,
        "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 28, 2023 at 02:47:02AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@...dia.com>
> > Sent: Wednesday, June 28, 2023 12:01 AM
> > 
> > On Tue, Jun 27, 2023 at 06:02:13AM +0000, Tian, Kevin wrote:
> > > > From: Nicolin Chen <nicolinc@...dia.com>
> > > > Sent: Tuesday, June 27, 2023 1:29 AM
> > > >
> > > > > I'm not sure whether the MSI region needs a special MSI type or
> > > > > just a general RESV_DIRECT type for 1:1 mapping, though.
> > > >
> > > > I don't quite get this part. Isn't MSI having IOMMU_RESV_MSI
> > > > and IOMMU_RESV_SW_MSI? Or does it juset mean we should report
> > > > the iommu_resv_type along with reserved regions in new ioctl?
> > > >
> > >
> > > Currently those are iommu internal types. When defining the new
> > > ioctl we need think about what are necessary presenting to the user.
> > >
> > > Probably just a list of reserved regions plus a flag to mark which
> > > one is SW_MSI? Except SW_MSI all other reserved region types
> > > just need the user to reserve them w/o knowing more detail.
> > 
> > I think I prefer the idea we just import the reserved regions from a
> > devid and do not expose any of this detail to userspace.
> > 
> > Kernel can make only the SW_MSI a mandatory cut out when the S2 is
> > attached.
> > 
> 
> I'm confused.
> 
> The VMM needs to know reserved regions per dev_id and report them
> to the guest.
> 
> And we have aligned on that reserved regions (except SW_MSI) should
> not be automatically added to S2 in nesting case. Then the VMM cannot
> rely on IOAS_IOVA_RANGES to identify the reserved regions.

We also said we need a way to load the reserved regions to create an
identity compatible version of the HWPT

So we have a model where the VMM will want to load in regions beyond
the currently attached device needs

> So there needs a new interface for the user to discover reserved regions
> per dev_id, within which the SW_MSI region should be marked out so
> identity mapping can be installed properly for it in S1.
> 
> Did I misunderstand your point in previous discussion?

This is another discussion, if the vmm needs this then we probably
need a new API to get it.

Jason