Re: [PATCH v0 12/15] x86/hyperv: Implement hyperv virtual iommu

Mukesh R
2026-01-27T03:02:41Z

On 1/26/26 07:57, Stanislav Kinsburskii wrote:
> On Fri, Jan 23, 2026 at 05:26:19PM -0800, Mukesh R wrote:
>> On 1/20/26 16:12, Stanislav Kinsburskii wrote:
>>> On Mon, Jan 19, 2026 at 10:42:27PM -0800, Mukesh R wrote:
>>>> From: Mukesh Rathor <mrathor@linux.microsoft.com>
>>>>
>>>> Add a new file to implement management of device domains, mapping and
>>>> unmapping of iommu memory, and other iommu_ops to fit within the VFIO
>>>> framework for PCI passthru on Hyper-V running Linux as root or L1VH
>>>> parent. This also implements direct attach mechanism for PCI passthru,
>>>> and it is also made to work within the VFIO framework.
>>>>
>>>> At a high level, during boot the hypervisor creates a default identity
>>>> domain and attaches all devices to it. This nicely maps to Linux iommu
>>>> subsystem IOMMU_DOMAIN_IDENTITY domain. As a result, Linux does not
>>>> need to explicitly ask Hyper-V to attach devices and do maps/unmaps
>>>> during boot. As mentioned previously, Hyper-V supports two ways to do
>>>> PCI passthru:
>>>>
>>>> 1. Device Domain: root must create a device domain in the hypervisor,
>>>> and do map/unmap hypercalls for mapping and unmapping guest RAM.
>>>> All hypervisor communications use device id of type PCI for
>>>> identifying and referencing the device.
>>>>
>>>> 2. Direct Attach: the hypervisor will simply use the guest's HW
>>>> page table for mappings, thus the host need not do map/unmap
>>>> device memory hypercalls. As such, direct attach passthru setup
>>>> during guest boot is extremely fast. A direct attached device
>>>> must be referenced via logical device id and not via the PCI
>>>> device id.
>>>>
>>>> At present, L1VH root/parent only supports direct attaches. Also direct
>>>> attach is default in non-L1VH cases because there are some significant
>>>> performance issues with device domain implementation currently for guests
>>>> with higher RAM (say more than 8GB), and that unfortunately cannot be
>>>> addressed in the short term.
>>>>
>>>
>>> <snip>
>>>
>
> <snip>
>
>>>> +static void hv_iommu_detach_dev(struct iommu_domain *immdom, struct device *dev)
>>>> +{
>>>> + struct pci_dev *pdev;
>>>> + struct hv_domain *hvdom = to_hv_domain(immdom);
>>>> +
>>>> + /* See the attach function, only PCI devices for now */
>>>> + if (!dev_is_pci(dev))
>>>> + return;
>>>> +
>>>> + if (hvdom->num_attchd == 0)
>>>> + pr_warn("Hyper-V: num_attchd is zero (%s)\n", dev_name(dev));
>>>> +
>>>> + pdev = to_pci_dev(dev);
>>>> +
>>>> + if (hvdom->attached_dom) {
>>>> + hv_iommu_det_dev_from_guest(hvdom, pdev);
>>>> +
>>>> + /* Do not reset attached_dom, hv_iommu_unmap_pages happens
>>>> + * next.
>>>> + */
>>>> + } else {
>>>> + hv_iommu_det_dev_from_dom(hvdom, pdev);
>>>> + }
>>>> +
>>>> + hvdom->num_attchd--;
>>>
>>> Shouldn't this be modified iff the detach succeeded?
>>
>> We want to still free the domain and not let it get stuck. The purpose
>> is more to make sure detach was called before domain free.
>>
>
> How can one debug subseqent errors if num_attchd is decremented
> unconditionally? In reality the device is left attached, but the related
> kernel metadata is gone.

Error is printed in case of failed detach. If there is panic, at least
you can get some info about the device. Metadata in hypervisor is
around if failed.

>>>> +}
>>>> +
>>>> +static int hv_iommu_add_tree_mapping(struct hv_domain *hvdom,
>>>> + unsigned long iova, phys_addr_t paddr,
>>>> + size_t size, u32 flags)
>>>> +{
>>>> + unsigned long irqflags;
>>>> + struct hv_iommu_mapping *mapping;
>>>> +
>>>> + mapping = kzalloc(sizeof(*mapping), GFP_ATOMIC);
>>>> + if (!mapping)
>>>> + return -ENOMEM;
>>>> +
>>>> + mapping->paddr = paddr;
>>>> + mapping->iova.start = iova;
>>>> + mapping->iova.last = iova + size - 1;
>>>> + mapping->flags = flags;
>>>> +
>>>> + spin_lock_irqsave(&hvdom->mappings_lock, irqflags);
>>>> + interval_tree_insert(&mapping->iova, &hvdom->mappings_tree);
>>>> + spin_unlock_irqrestore(&hvdom->mappings_lock, irqflags);
>>>> +
>>>> + return 0;
>>>> +}
>>>> +
>>>> +static size_t hv_iommu_del_tree_mappings(struct hv_domain *hvdom,
>>>> + unsigned long iova, size_t size)
>>>> +{
>>>> + unsigned long flags;
>>>> + size_t unmapped = 0;
>>>> + unsigned long last = iova + size - 1;
>>>> + struct hv_iommu_mapping *mapping = NULL;
>>>> + struct interval_tree_node *node, *next;
>>>> +
>>>> + spin_lock_irqsave(&hvdom->mappings_lock, flags);
>>>> + next = interval_tree_iter_first(&hvdom->mappings_tree, iova, last);
>>>> + while (next) {
>>>> + node = next;
>>>> + mapping = container_of(node, struct hv_iommu_mapping, iova);
>>>> + next = interval_tree_iter_next(node, iova, last);
>>>> +
>>>> + /* Trying to split a mapping? Not supported for now. */
>>>> + if (mapping->iova.start < iova)
>>>> + break;
>>>> +
>>>> + unmapped += mapping->iova.last - mapping->iova.start + 1;
>>>> +
>>>> + interval_tree_remove(node, &hvdom->mappings_tree);
>>>> + kfree(mapping);
>>>> + }
>>>> + spin_unlock_irqrestore(&hvdom->mappings_lock, flags);
>>>> +
>>>> + return unmapped;
>>>> +}
>>>> +
>>>> +/* Return: must return exact status from the hypercall without changes */
>>>> +static u64 hv_iommu_map_pgs(struct hv_domain *hvdom,
>>>> + unsigned long iova, phys_addr_t paddr,
>>>> + unsigned long npages, u32 map_flags)
>>>> +{
>>>> + u64 status;
>>>> + int i;
>>>> + struct hv_input_map_device_gpa_pages *input;
>>>> + unsigned long flags, pfn = paddr >> HV_HYP_PAGE_SHIFT;
>>>> +
>>>> + local_irq_save(flags);
>>>> + input = *this_cpu_ptr(hyperv_pcpu_input_arg);
>>>> + memset(input, 0, sizeof(*input));
>>>> +
>>>> + input->device_domain.partition_id = HV_PARTITION_ID_SELF;
>>>> + input->device_domain.domain_id.type = HV_DEVICE_DOMAIN_TYPE_S2;
>>>> + input->device_domain.domain_id.id = hvdom->domid_num;
>>>> + input->map_flags = map_flags;
>>>> + input->target_device_va_base = iova;
>>>> +
>>>> + pfn = paddr >> HV_HYP_PAGE_SHIFT;
>>>> + for (i = 0; i < npages; i++, pfn++)
>>>> + input->gpa_page_list[i] = pfn;
>>>> +
>>>> + status = hv_do_rep_hypercall(HVCALL_MAP_DEVICE_GPA_PAGES, npages, 0,
>>>> + input, NULL);
>>>> +
>>>> + local_irq_restore(flags);
>>>> + return status;
>>>> +}
>>>> +
>>>> +/*
>>>> + * The core VFIO code loops over memory ranges calling this function with
>>>> + * the largest size from HV_IOMMU_PGSIZES. cond_resched() is in vfio_iommu_map.
>>>> + */
>>>> +static int hv_iommu_map_pages(struct iommu_domain *immdom, ulong iova,
>>>> + phys_addr_t paddr, size_t pgsize, size_t pgcount,
>>>> + int prot, gfp_t gfp, size_t *mapped)
>>>> +{
>>>> + u32 map_flags;
>>>> + int ret;
>>>> + u64 status;
>>>> + unsigned long npages, done = 0;
>>>> + struct hv_domain *hvdom = to_hv_domain(immdom);
>>>> + size_t size = pgsize * pgcount;
>>>> +
>>>> + map_flags = HV_MAP_GPA_READABLE; /* required */
>>>> + map_flags |= prot & IOMMU_WRITE ? HV_MAP_GPA_WRITABLE : 0;
>>>> +
>>>> + ret = hv_iommu_add_tree_mapping(hvdom, iova, paddr, size, map_flags);
>>>> + if (ret)
>>>> + return ret;
>>>> +
>>>> + if (hvdom->attached_dom) {
>>>> + *mapped = size;
>>>> + return 0;
>>>> + }
>>>> +
>>>> + npages = size >> HV_HYP_PAGE_SHIFT;
>>>> + while (done < npages) {
>>>> + ulong completed, remain = npages - done;
>>>> +
>>>> + status = hv_iommu_map_pgs(hvdom, iova, paddr, remain,
>>>> + map_flags);
>>>> +
>>>> + completed = hv_repcomp(status);
>>>> + done = done + completed;
>>>> + iova = iova + (completed << HV_HYP_PAGE_SHIFT);
>>>> + paddr = paddr + (completed << HV_HYP_PAGE_SHIFT);
>>>> +
>>>> + if (hv_result(status) == HV_STATUS_INSUFFICIENT_MEMORY) {
>>>> + ret = hv_call_deposit_pages(NUMA_NO_NODE,
>>>> + hv_current_partition_id,
>>>> + 256);
>>>> + if (ret)
>>>> + break;
>>>> + }
>>>> + if (!hv_result_success(status))
>>>> + break;
>>>> + }
>>>> +
>>>> + if (!hv_result_success(status)) {
>>>> + size_t done_size = done << HV_HYP_PAGE_SHIFT;
>>>> +
>>>> + hv_status_err(status, "pgs:%lx/%lx iova:%lx\n",
>>>> + done, npages, iova);
>>>> + /*
>>>> + * lookup tree has all mappings [0 - size-1]. Below unmap will
>>>> + * only remove from [0 - done], we need to remove second chunk
>>>> + * [done+1 - size-1].
>>>> + */
>>>> + hv_iommu_del_tree_mappings(hvdom, iova, size - done_size);
>>>> + hv_iommu_unmap_pages(immdom, iova - done_size, pgsize,
>>>> + done, NULL);
>>>> + if (mapped)
>>>> + *mapped = 0;
>>>> + } else
>>>> + if (mapped)
>>>> + *mapped = size;
>>>> +
>>>> + return hv_result_to_errno(status);
>>>> +}
>>>> +
>>>> +static size_t hv_iommu_unmap_pages(struct iommu_domain *immdom, ulong iova,
>>>> + size_t pgsize, size_t pgcount,
>>>> + struct iommu_iotlb_gather *gather)
>>>> +{
>>>> + unsigned long flags, npages;
>>>> + struct hv_input_unmap_device_gpa_pages *input;
>>>> + u64 status;
>>>> + struct hv_domain *hvdom = to_hv_domain(immdom);
>>>> + size_t unmapped, size = pgsize * pgcount;
>>>> +
>>>> + unmapped = hv_iommu_del_tree_mappings(hvdom, iova, size);
>>>> + if (unmapped < size)
>>>> + pr_err("%s: could not delete all mappings (%lx:%lx/%lx)\n",
>>>> + __func__, iova, unmapped, size);
>>>> +
>>>> + if (hvdom->attached_dom)
>>>> + return size;
>>>> +
>>>> + npages = size >> HV_HYP_PAGE_SHIFT;
>>>> +
>>>> + local_irq_save(flags);
>>>> + input = *this_cpu_ptr(hyperv_pcpu_input_arg);
>>>> + memset(input, 0, sizeof(*input));
>>>> +
>>>> + input->device_domain.partition_id = HV_PARTITION_ID_SELF;
>>>> + input->device_domain.domain_id.type = HV_DEVICE_DOMAIN_TYPE_S2;
>>>> + input->device_domain.domain_id.id = hvdom->domid_num;
>>>> + input->target_device_va_base = iova;
>>>> +
>>>> + status = hv_do_rep_hypercall(HVCALL_UNMAP_DEVICE_GPA_PAGES, npages,
>>>> + 0, input, NULL);
>>>> + local_irq_restore(flags);
>>>> +
>>>> + if (!hv_result_success(status))
>>>> + hv_status_err(status, "\n");
>>>> +
>>>
>>> There is some inconsistency in namings and behaviour of paired
>>> functions:
>>> 1. The pair of hv_iommu_unmap_pages is called hv_iommu_map_pgs
>>
>> The pair of hv_iommu_unmap_pages is hv_iommu_map_pages right above.
>> hv_iommu_map_pgs could be renamed to hv_iommu_map_pgs_hcall I suppose.
>>
>
> Hv_iommu_map_pages is a wrapper around hv_iommu_map_pgs while
> hv_iommu_unmap_pages is a wrapper around the correspodning hypercall.
> That's the inconsistency I meant.

Unmap does not need intermediate function.

>>> 2. hv_iommu_map_pgs doesn't print status in case of error.

We print error upon its failure in hv_iommu_map_pages():

if (!hv_result_success(status)) {
size_t done_size = done << HV_HYP_PAGE_SHIFT;
hv_status_err(status, "pgs:%lx/%lx iova:%lx\n",
done, npages, iova);

>> it does:
>> hv_status_err(status, "\n"); <==============
>
> It does not. I guess you are confusing it with some other function.
> Here is the function:
>
>>
>>
>>> It would be much better to keep this code consistent.
>>>
>>>> + return unmapped;
>>>> +}
>>>> +
>>>> +static phys_addr_t hv_iommu_iova_to_phys(struct iommu_domain *immdom,
>>>> + dma_addr_t iova)
>>>> +{
>>>> + u64 paddr = 0;
>>>> + unsigned long flags;
>>>> + struct hv_iommu_mapping *mapping;
>>>> + struct interval_tree_node *node;
>>>> + struct hv_domain *hvdom = to_hv_domain(immdom);
>>>> +
>>>> + spin_lock_irqsave(&hvdom->mappings_lock, flags);
>>>> + node = interval_tree_iter_first(&hvdom->mappings_tree, iova, iova);
>>>> + if (node) {
>>>> + mapping = container_of(node, struct hv_iommu_mapping, iova);
>>>> + paddr = mapping->paddr + (iova - mapping->iova.start);
>>>> + }
>>>> + spin_unlock_irqrestore(&hvdom->mappings_lock, flags);
>>>> +
>>>> + return paddr;
>>>> +}
>>>> +
>>>> +/*
>>>> + * Currently, hypervisor does not provide list of devices it is using
>>>> + * dynamically. So use this to allow users to manually specify devices that
>>>> + * should be skipped. (eg. hypervisor debugger using some network device).
>>>> + */
>>>> +static struct iommu_device *hv_iommu_probe_device(struct device *dev)
>>>> +{
>>>> + if (!dev_is_pci(dev))
>>>> + return ERR_PTR(-ENODEV);
>>>> +
>>>> + if (pci_devs_to_skip && *pci_devs_to_skip) {
>>>> + int rc, pos = 0;
>>>> + int parsed;
>>>> + int segment, bus, slot, func;
>>>> + struct pci_dev *pdev = to_pci_dev(dev);
>>>> +
>>>> + do {
>>>> + parsed = 0;
>>>> +
>>>> + rc = sscanf(pci_devs_to_skip + pos, " (%x:%x:%x.%x) %n",
>>>> + &segment, &bus, &slot, &func, &parsed);
>>>> + if (rc)
>>>> + break;
>>>> + if (parsed <= 0)
>>>> + break;
>>>> +
>>>> + if (pci_domain_nr(pdev->bus) == segment &&
>>>> + pdev->bus->number == bus &&
>>>> + PCI_SLOT(pdev->devfn) == slot &&
>>>> + PCI_FUNC(pdev->devfn) == func) {
>>>> +
>>>> + dev_info(dev, "skipped by Hyper-V IOMMU\n");
>>>> + return ERR_PTR(-ENODEV);
>>>> + }
>>>> + pos += parsed;
>>>> +
>>>> + } while (pci_devs_to_skip[pos]);
>>>> + }
>>>> +
>>>> + /* Device will be explicitly attached to the default domain, so no need
>>>> + * to do dev_iommu_priv_set() here.
>>>> + */
>>>> +
>>>> + return &hv_virt_iommu;
>>>> +}
>>>> +
>>>> +static void hv_iommu_probe_finalize(struct device *dev)
>>>> +{
>>>> + struct iommu_domain *immdom = iommu_get_domain_for_dev(dev);
>>>> +
>>>> + if (immdom && immdom->type == IOMMU_DOMAIN_DMA)
>>>> + iommu_setup_dma_ops(dev);
>>>> + else
>>>> + set_dma_ops(dev, NULL);
>>>> +}
>>>> +
>>>> +static void hv_iommu_release_device(struct device *dev)
>>>> +{
>>>> + struct hv_domain *hvdom = dev_iommu_priv_get(dev);
>>>> +
>>>> + /* Need to detach device from device domain if necessary. */
>>>> + if (hvdom)
>>>> + hv_iommu_detach_dev(&hvdom->iommu_dom, dev);
>>>> +
>>>> + dev_iommu_priv_set(dev, NULL);
>>>> + set_dma_ops(dev, NULL);
>>>> +}
>>>> +
>>>> +static struct iommu_group *hv_iommu_device_group(struct device *dev)
>>>> +{
>>>> + if (dev_is_pci(dev))
>>>> + return pci_device_group(dev);
>>>> + else
>>>> + return generic_device_group(dev);
>>>> +}
>>>> +
>>>> +static int hv_iommu_def_domain_type(struct device *dev)
>>>> +{
>>>> + /* The hypervisor always creates this by default during boot */
>>>> + return IOMMU_DOMAIN_IDENTITY;
>>>> +}
>>>> +
>>>> +static struct iommu_ops hv_iommu_ops = {
>>>> + .capable = hv_iommu_capable,
>>>> + .domain_alloc_identity = hv_iommu_domain_alloc_identity,
>>>> + .domain_alloc_paging = hv_iommu_domain_alloc_paging,
>>>> + .probe_device = hv_iommu_probe_device,
>>>> + .probe_finalize = hv_iommu_probe_finalize,
>>>> + .release_device = hv_iommu_release_device,
>>>> + .def_domain_type = hv_iommu_def_domain_type,
>>>> + .device_group = hv_iommu_device_group,
>>>> + .default_domain_ops = &(const struct iommu_domain_ops) {
>>>> + .attach_dev = hv_iommu_attach_dev,
>>>> + .map_pages = hv_iommu_map_pages,
>>>> + .unmap_pages = hv_iommu_unmap_pages,
>>>> + .iova_to_phys = hv_iommu_iova_to_phys,
>>>> + .free = hv_iommu_domain_free,
>>>> + },
>>>> + .owner = THIS_MODULE,
>>>> +};
>>>> +
>>>> +static void __init hv_initialize_special_domains(void)
>>>> +{
>>>> + hv_def_identity_dom.iommu_dom.geometry = default_geometry;
>>>> + hv_def_identity_dom.domid_num = HV_DEVICE_DOMAIN_ID_S2_DEFAULT; /* 0 */
>>>
>>> hv_def_identity_dom is a static global variable.
>>> Why not initialize hv_def_identity_dom upon definition instead of
>>> introducing a new function?
>>
>> Originally, it was function. I changed it static, but during 6.6
>> review I changed it back to function. I can't remember why, but is
>> pretty harmless. We may add more domains, for example null domain to the
>> initilization in future.
>>
>>>> +}
>>>> +
>>>> +static int __init hv_iommu_init(void)
>>>> +{
>>>> + int ret;
>>>> + struct iommu_device *iommup = &hv_virt_iommu;
>>>> +
>>>> + if (!hv_is_hyperv_initialized())
>>>> + return -ENODEV;
>>>> +
>>>> + ret = iommu_device_sysfs_add(iommup, NULL, NULL, "%s", "hyperv-iommu");
>>>> + if (ret) {
>>>> + pr_err("Hyper-V: iommu_device_sysfs_add failed: %d\n", ret);
>>>> + return ret;
>>>> + }
>>>> +
>>>> + /* This must come before iommu_device_register because the latter calls
>>>> + * into the hooks.
>>>> + */
>>>> + hv_initialize_special_domains();
>>>> +
>>>> + ret = iommu_device_register(iommup, &hv_iommu_ops, NULL);
>>>
>>> It looks weird to initialize an object after creating sysfs entries for
>>> it.
>>> It should be the other way around.
>>
>> Not sure if it should be, much easier to remove sysfs entry than other
>> cleanup, even tho iommu_device_unregister is there. I am sure we'll add
>> more code here, probably why it was originally done this way.
>>
>
> Sysfs provides user space access to kernel objects. If the object is not
> initialized, it's not only a useless sysfs entry, but also a potential
> cause for kernel panic if user space will try to access this entry
> before the object is initialized.

I hear you... but,
o there is nothing under sysfs to be accessed when created
o it is during boot
o it should almost never fail...
o iommu_device_sysfs_remove is much more light weight than
iommu_device_unregister
o i expect more to be added there as we enhance it

Thanks,
-Mukesh

> Thanks,
> Stanislav
>
>
>> Thanks,
>> -Mukesh
>>
>>
>> ... snip........