linux-kernel - Re: [PATCH v1 01/14] iommu: Add iommu_get_unmanaged

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Date:   Fri, 10 Mar 2023 11:33:00 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Eric Auger <eric.auger@...hat.com>
Cc:     Nicolin Chen <nicolinc@...dia.com>, robin.murphy@....com,
        will@...nel.org, kevin.tian@...el.com, baolu.lu@...ux.intel.com,
        joro@...tes.org, shameerali.kolothum.thodi@...wei.com,
        jean-philippe@...aro.org, linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 01/14] iommu: Add iommu_get_unmanaged_domain helper

On Fri, Mar 10, 2023 at 11:14:59AM +0100, Eric Auger wrote:
> Hi Nicolin,
> 
> On 3/9/23 11:53, Nicolin Chen wrote:
> > The nature of ITS virtualization on ARM is done via hypercalls, so kernel
> > handles all IOVA mappings for the MSI doorbell in iommu_dma_prepare_msi()
> > and iommu_dma_compose_msi_msg(). The current virtualization solution with
> > a 2-stage nested translation setup is to do 1:1 IOVA mappings at stage-1
> Note that if we still intend to use that trick there is a known issue at
> kernel side that needs to be fixed.
> 
> ARM DEN 0049E.b IORT specification mandates that when
> RMRs are present, the OS must preserve PCIe configuration
> performed by the boot FW.

This limitation doesn't seem necessary for this MSI stuff?

What is it for?

Jason