Date:   Fri, 10 Mar 2023 11:33:00 -0400
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Eric Auger <eric.auger@redhat.com>
Cc:     Nicolin Chen <nicolinc@nvidia.com>, robin.murphy@arm.com,
        will@kernel.org, kevin.tian@intel.com, baolu.lu@linux.intel.com,
        joro@8bytes.org, shameerali.kolothum.thodi@huawei.com,
        jean-philippe@linaro.org, linux-arm-kernel@lists.infradead.org,
        iommu@lists.linux.dev, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1 01/14] iommu: Add iommu_get_unmanaged_domain helper
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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