linux-kernel - Re: [RFC v1 5/5] iommu/hyperv: Add para-virtualized IOMMU support for Hyper-V guest

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Message-ID: <u7j6pq2yhbxsa76p4lyidozjrrokpb76pul7foxfrbqxyf3vgz@cfr6zmej3nhs>
Date: Thu, 11 Dec 2025 16:41:23 +0800
From: Yu Zhang <zhangyu1@...ux.microsoft.com>
To: Easwar Hariharan <easwar.hariharan@...ux.microsoft.com>
Cc: linux-kernel@...r.kernel.org, linux-hyperv@...r.kernel.org, 
	iommu@...ts.linux.dev, linux-pci@...r.kernel.org, kys@...rosoft.com, 
	haiyangz@...rosoft.com, wei.liu@...nel.org, decui@...rosoft.com, lpieralisi@...nel.org, 
	kwilczynski@...nel.org, mani@...nel.org, robh@...nel.org, bhelgaas@...gle.com, 
	arnd@...db.de, joro@...tes.org, will@...nel.org, robin.murphy@....com, 
	jacob.pan@...ux.microsoft.com, nunodasneves@...ux.microsoft.com, mrathor@...ux.microsoft.com, 
	mhklinux@...look.com, peterz@...radead.org, linux-arch@...r.kernel.org
Subject: Re: [RFC v1 5/5] iommu/hyperv: Add para-virtualized IOMMU support
 for Hyper-V guest

On Wed, Dec 10, 2025 at 09:15:18AM -0800, Easwar Hariharan wrote:
> On 12/8/2025 9:11 PM, 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 elemenating the VM exits for guest IOMMU mapping
> > operations.
> > 
> > For guest IOMMU unmapping operations, VM exits to perform the IOTLB
> > flush(and possibly the device TLB flush) is still unavoidable. For
> > now, HVCALL_FLUSH_DEVICE_DOMAIN	is used to implement a domain-selective
> > IOTLB flush. New hypercalls for finer-grained hypercall will be provided
> > in future patches.
> > 
> > Co-developed-by: Wei Liu <wei.liu@...nel.org>
> > Signed-off-by: Wei Liu <wei.liu@...nel.org>
> > Co-developed-by: Jacob Pan <jacob.pan@...ux.microsoft.com>
> > Signed-off-by: Jacob Pan <jacob.pan@...ux.microsoft.com>
> > Co-developed-by: Easwar Hariharan <easwar.hariharan@...ux.microsoft.com>
> > Signed-off-by: Easwar Hariharan <easwar.hariharan@...ux.microsoft.com>
> > Signed-off-by: Yu Zhang <zhangyu1@...ux.microsoft.com>
> > ---
> >  drivers/iommu/hyperv/Kconfig  |  14 +
> >  drivers/iommu/hyperv/Makefile |   1 +
> >  drivers/iommu/hyperv/iommu.c  | 608 ++++++++++++++++++++++++++++++++++
> >  drivers/iommu/hyperv/iommu.h  |  53 +++
> >  4 files changed, 676 insertions(+)
> >  create mode 100644 drivers/iommu/hyperv/iommu.c
> >  create mode 100644 drivers/iommu/hyperv/iommu.h
> > 
> 
> <snip>
> 
> > +
> > +static int __init hv_iommu_init(void)
> > +{
> > +	int ret = 0;
> > +	struct hv_iommu_dev *hv_iommu = NULL;
> > +	struct hv_output_get_iommu_capabilities hv_iommu_cap = {0};
> > +
> > +	if (no_iommu || iommu_detected)
> > +		return -ENODEV;
> > +
> > +	if (!hv_is_hyperv_initialized())
> > +		return -ENODEV;
> > +
> > +	if (hv_iommu_detect(&hv_iommu_cap) ||
> > +	    !hv_iommu_present(hv_iommu_cap.iommu_cap) ||
> > +	    !hv_iommu_s1_domain_supported(hv_iommu_cap.iommu_cap))
> > +		return -ENODEV;
> > +
> > +	iommu_detected = 1;
> > +	pci_request_acs();
> > +
> > +	hv_iommu = kzalloc(sizeof(*hv_iommu), GFP_KERNEL);
> > +	if (!hv_iommu)
> > +		return -ENOMEM;
> > +
> > +	hv_init_iommu_device(hv_iommu, &hv_iommu_cap);
> > +
> > +	ret = hv_initialize_static_domains();
> > +	if (ret) {
> > +		pr_err("hv_initialize_static_domains failed: %d\n", ret);
> > +		goto err_sysfs_remove;
> 
> This should be goto err_free since we haven't done the sysfs_add yet
> 
> > +	}
> > +
> > +	ret = iommu_device_sysfs_add(&hv_iommu->iommu, NULL, NULL, "%s", "hv-iommu");
> > +	if (ret) {
> > +		pr_err("iommu_device_sysfs_add failed: %d\n", ret);
> > +		goto err_free;
> 
> And this should be probably a goto delete_static_domains that cleans up the allocated static
> domains...
> 

Nice catch. And thanks! :)

Yu