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Date: Thu, 8 Jan 2026 18:45:52 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Yu Zhang <zhangyu1@...ux.microsoft.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-hyperv@...r.kernel.org"
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Subject: RE: [RFC v1 0/5] Hyper-V: Add para-virtualized IOMMU support for
 Linux guests

From: Yu Zhang <zhangyu1@...ux.microsoft.com> Sent: Monday, December 8, 2025 9:11 PM
> 
> This patch series introduces a para-virtualized IOMMU driver for
> Linux guests running on Microsoft Hyper-V. The primary objective
> is to enable hardware-assisted DMA isolation and scalable device

Is there any particular meaning for the qualifier "scalable" vs. just
"device assignment"? I just want to understand what you are getting
at.

> assignment for Hyper-V child partitions, bypassing the performance
> overhead and complexity associated with emulated IOMMU hardware.
> 
> The driver implements the following core functionality:
> *   Hypercall-based Enumeration
>     Unlike traditional ACPI-based discovery (e.g., DMAR/IVRS),
>     this driver enumerates the Hyper-V IOMMU capabilities directly
>     via hypercalls. This approach allows the guest to discover
>     IOMMU presence and features without requiring specific virtual
>     firmware extensions or modifications.
> 
> *   Domain Management
>     The driver manages IOMMU domains through a new set of Hyper-V
>     hypercall interfaces, handling domain allocation, attachment,
>     and detachment for endpoint devices.
> 
> *   IOTLB Invalidation
>     IOTLB invalidation requests are marshaled and issued to the
>     hypervisor through the same hypercall mechanism.
> 
> *   Nested Translation Support
>     This implementation leverages guest-managed stage-1 I/O page
>     tables nested with host stage-2 translations. It is built
>     upon the consolidated IOMMU page table framework designed by
>     Jason Gunthorpe [1]. This design eliminates the need for complex
>     emulation during map operations and ensures scalability across
>     different architectures.
> 
> Implementation Notes:
> *   Architecture Independence
>     While the current implementation only supports x86 platforms (Intel
>     VT-d and AMD IOMMU), the driver design aims to be as architecture-
>     agnostic as possible. To achieve this, initialization occurs via
>     `device_initcall` rather than `x86_init.iommu.iommu_init`, and shutdown
>     is handled via `syscore_ops` instead of `x86_platform.iommu_shutdown`.
> 
> *   MSI Region Handling
>     In this RFC, the hardware MSI region is hard-coded to the standard
>     x86 interrupt range (0xfee00000 - 0xfeefffff). Future updates may
>     allow this configuration to be queried via hypercalls if new hardware
>     platforms are to be supported.
> 
> *   Reserved Regions (RMRR)
>     There is currently no requirement to support assigned devices with
>     ACPI RMRR limitations. Consequently, this patch series does not specify
>     or query reserved memory regions.
> 
> Testing:
> This series has been validated using dmatest with Intel DSA devices
> assigned to the child partition. The tests confirmed successful DMA
> transactions under the para-virtualized IOMMU.
> 
> Future Work:
> *   Page-selective IOTLB Invalidation
>     The current implementation relies on full-domain flushes. Support
>     for page-selective invalidation is planned for a future series.
> 
> *   Advanced Features
>     Support for vSVA and virtual PRI will be addressed in subsequent
>     updates.
> 
> *   Root Partition Co-existence
>     Ensure compatibility with the distinct para-virtualized IOMMU driver
>     used by Hyper-V's Linux root partition, in which the DMA remapping
>     is not achieved by stage-1 IO page tables and another set of iommu
>     ops is provided.
> 
> [1] https://github.com/jgunthorpe/linux/tree/iommu_pt_all 
> 
> Easwar Hariharan (2):
>   PCI: hv: Create and export hv_build_logical_dev_id()
>   iommu: Move Hyper-V IOMMU driver to its own subdirectory
> 
> Wei Liu (1):
>   hyperv: Introduce new hypercall interfaces used by Hyper-V guest IOMMU
> 
> Yu Zhang (2):
>   hyperv: allow hypercall output pages to be allocated for child
>     partitions
>   iommu/hyperv: Add para-virtualized IOMMU support for Hyper-V guest
> 
>  drivers/hv/hv_common.c                        |  21 +-
>  drivers/iommu/Kconfig                         |  10 +-
>  drivers/iommu/Makefile                        |   2 +-
>  drivers/iommu/hyperv/Kconfig                  |  24 +
>  drivers/iommu/hyperv/Makefile                 |   3 +
>  drivers/iommu/hyperv/iommu.c                  | 608 ++++++++++++++++++
>  drivers/iommu/hyperv/iommu.h                  |  53 ++
>  .../irq_remapping.c}                          |   2 +-
>  drivers/pci/controller/pci-hyperv.c           |  28 +-
>  include/asm-generic/mshyperv.h                |   2 +
>  include/hyperv/hvgdk_mini.h                   |   8 +
>  include/hyperv/hvhdk_mini.h                   | 123 ++++
>  12 files changed, 850 insertions(+), 34 deletions(-)
>  create mode 100644 drivers/iommu/hyperv/Kconfig
>  create mode 100644 drivers/iommu/hyperv/Makefile
>  create mode 100644 drivers/iommu/hyperv/iommu.c
>  create mode 100644 drivers/iommu/hyperv/iommu.h
>  rename drivers/iommu/{hyperv-iommu.c => hyperv/irq_remapping.c} (99%)
> 
> --
> 2.49.0

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