linux-kernel - Re: [RFC PATCH 00/21] iommu/amd: Introduce support for HW accelerated vIOMMU w/ nested page table

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Message-ID: <ZJYjOor3TKSeIo7F@nvidia.com>
Date:   Fri, 23 Jun 2023 19:56:58 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Suthikulpanit, Suravee" <suravee.suthikulpanit@....com>
Cc:     linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
        kvm@...r.kernel.org, joro@...tes.org, robin.murphy@....com,
        yi.l.liu@...el.com, alex.williamson@...hat.com,
        nicolinc@...dia.com, baolu.lu@...ux.intel.com,
        eric.auger@...hat.com, pandoh@...gle.com, kumaranand@...gle.com,
        jon.grimm@....com, santosh.shukla@....com, vasant.hegde@....com,
        jay.chen@....com, joseph.chung@....com
Subject: Re: [RFC PATCH 00/21] iommu/amd: Introduce support for HW
 accelerated vIOMMU w/ nested page table

On Fri, Jun 23, 2023 at 03:05:06PM -0700, Suthikulpanit, Suravee wrote:

> For example, an AMD IOMMU hardware is normally listed as a PCI device (e.g.
> PCI ID 00:00.2). To setup IOMMU PAS for this IOMMU instance, the IOMMU
> driver allocate an IOMMU v1 page table for this device, which contains PAS
> mapping.

So it is just system dram?
 
> The IOMMU hardware use the PAS for storing Guest IOMMU information such as
> Guest MMIOs, DevID Mapping Table, DomID Mapping Table, and Guest
> Command/Event/PPR logs.

Why does it have to be in kernel memory?

Why not store the whole thing in user mapped memory and have the VMM
manipulate it directly?

Jason