linux-kernel - Re: [PATCH v3 07/17] iommufd: Add IOMMU_RESV_IOVA

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Date:   Fri, 28 Jul 2023 14:16:50 -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 v3 07/17] iommufd: Add IOMMU_RESV_IOVA_RANGES

On Fri, Jul 28, 2023 at 10:07:58AM +0000, Tian, Kevin wrote:
> > From: Liu, Yi L <yi.l.liu@...el.com>
> > Sent: Monday, July 24, 2023 7:04 PM
> > 
> > This reports device's reserved IOVA regions to userspace. This is needed
> > in the nested translation as userspace owns stage-1 HWPT, and userspace
> > needs to exclude the reserved IOVA regions in the stage-1 HWPT hence
> > exclude
> > them in the device's DMA address space.
> > 
> > This can also be used to figure out allowed IOVAs of an IOAS.
> 
> We may need a special type to mark SW_MSI since it requires identity
> mapping in stage-1 instead of being reserved.

Only the kernel can do this, so there is no action for user space to
take beyond knowing that is is not mappable IOVA.

The merit for "SW_MSI" may be to inform the rest of the system about
the IOVA of the ITS page, but with the current situation that isn't
required since only the kernel needs that information.

I think the long term way forward is to somehow arrange for the SW_MSI
to not become mapped when creating the parent HWPT and instead cause
the ITS page to be mapped through some explicit IOCTL.

Jason