linux-kernel - Re: [PATCH v12 03/13] vfio: VFIO_IOMMU_SET_MSI

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Date:   Fri, 5 Mar 2021 11:45:50 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Eric Auger <eric.auger@...hat.com>
Cc:     eric.auger.pro@...il.com, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu, will@...nel.org, maz@...nel.org,
        robin.murphy@....com, joro@...tes.org, alex.williamson@...hat.com,
        tn@...ihalf.com, zhukeqian1@...wei.com,
        jacob.jun.pan@...ux.intel.com, yi.l.liu@...el.com,
        wangxingang5@...wei.com, jiangkunkun@...wei.com,
        zhangfei.gao@...aro.org, zhangfei.gao@...il.com,
        vivek.gautam@....com, shameerali.kolothum.thodi@...wei.com,
        yuzenghui@...wei.com, nicoleotsuka@...il.com,
        lushenming@...wei.com, vsethi@...dia.com
Subject: Re: [PATCH v12 03/13] vfio: VFIO_IOMMU_SET_MSI_BINDING

Hi,

On Tue, Feb 23, 2021 at 10:06:15PM +0100, Eric Auger wrote:
> This patch adds the VFIO_IOMMU_SET_MSI_BINDING ioctl which aim
> to (un)register the guest MSI binding to the host. This latter
> then can use those stage 1 bindings to build a nested stage
> binding targeting the physical MSIs.

Now that RMR is in the IORT spec, could it be used for the nested MSI
problem?  For virtio-iommu tables I was planning to do it like this:

MSI is mapped at stage-2 with an arbitrary IPA->doorbell PA. We report
this IPA to userspace through iommu_groups/X/reserved_regions. No change
there. Then to the guest we report a reserved identity mapping at IPA
(using RMR, an equivalent DT binding, or probed RESV_MEM for
virtio-iommu). The guest creates that mapping at stage-1, and that's it.
Unless I overlooked something we'd only reuse existing infrastructure and
avoid the SET_MSI_BINDING interface.

Thanks,
Jean