Re: [PATCH v3 4/5] iommu/hyperv: Add para-virtualized IOMMU support for Hyper-V guest

On Thu, Aug 13, 2026 at 11:02:07AM -0700, Easwar Hariharan wrote:
> On 8/11/2026 08:50, Yu Zhang wrote:
> > Add a para-virtualized IOMMU driver for Linux guests running on Hyper-V.
> > This driver implements stage-1 IO translation within the guest OS.
> > It integrates with the Linux IOMMU core, utilizing Hyper-V hypercalls
> > for:
> >  - Capability discovery
> >  - Domain allocation, configuration, and deallocation
> >  - Device attachment and detachment
> >  - IOTLB invalidation
> > 
> > The driver constructs x86-compatible stage-1 IO page tables in the
> > guest memory using consolidated IO page table helpers. This allows
> > the guest to manage stage-1 translations independently of vendor-
> > specific drivers (like Intel VT-d or AMD IOMMU).
> > 
> > Hyper-V consumes this stage-1 IO page table when a device domain is
> > created and configured, and nests it with the host's stage-2 IO page
> > tables, therefore eliminating the VM exits for guest IOMMU mapping
> > operations. For unmapping operations, VM exits to perform the IOTLB
> > flush are still unavoidable.
> > 
> > Guest hibernation and resume are not supported by this initial
> > implementation. The guest-owned stage-1 page tables reside in guest
> > memory and are preserved in the hibernation image. However, the image
> > does not by itself preserve the Hyper-V device-domain objects or their
> > device attachments, and the current pvIOMMU interface does not define
> > whether or how Hyper-V restores that state.
> > 
> > To identify a device in its hypercall interface, the driver looks up the
> > logical device ID prefix registered for the device's PCI domain (see the
> > logical device ID registry in hv_common.c) and combines it with the PCI
> > function number of the endpoint device.
> > 
> > Co-developed-by: Wei Liu <[email protected]>
> > Signed-off-by: Wei Liu <[email protected]>
> > Co-developed-by: Easwar Hariharan <[email protected]>
> > Signed-off-by: Easwar Hariharan <[email protected]>
> > Signed-off-by: Yu Zhang <[email protected]>
> > ---
> >  arch/x86/hyperv/hv_init.c             |   4 +
> >  arch/x86/include/asm/mshyperv.h       |   4 +
> >  drivers/iommu/Kconfig                 |   1 +
> >  drivers/iommu/hyperv/Kconfig          |  16 +
> >  drivers/iommu/hyperv/Makefile         |   1 +
> >  drivers/iommu/hyperv/hv-iommu-guest.c | 632 ++++++++++++++++++++++++++
> >  drivers/iommu/hyperv/iommu.h          |  50 ++
> >  7 files changed, 708 insertions(+)
> >  create mode 100644 drivers/iommu/hyperv/Kconfig
> >  create mode 100644 drivers/iommu/hyperv/hv-iommu-guest.c
> >  create mode 100644 drivers/iommu/hyperv/iommu.h
> > 
> Seems like this is missing a change to MAINTAINERS. I'd suggest making a new section
> and adding both the IOMMU and Hyper-V lists. As it stands, the IOMMU list will get no
> notification of changes to this driver.
> 

Thanks for your remind, Easwar. 

We do intend to maintain the Linux guest pvIOMMU driver. I am just not
sure whether it needs a separate section in the MAINTAINERS file, since
drivers/iommu/hyperv/ is already covered by both the Hyper-V and generic
IOMMU entries, and get_maintainer.pl returns both mailing lists and sets
of maintainers. Therefore I don't see a notification gap that requires a
separate entry.
                                                                                                                                                        
Do the Hyper-V and IOMMU maintainers have a preference here?

B.R.
Yu
> Thanks,
> Easwar (he/him)
>