Re: [PATCH V4 01/10] iommufd: Support a HWPT without an iommu driver for noiommu

Yi Liu
2026-04-28T07:37:32Z

Hi Jacob,

On 4/15/26 05:14, Jacob Pan wrote:
> From: Jason Gunthorpe <jgg@nvidia.com>
>
> Create just a little part of a real iommu driver, enough to
> slot in under the dev_iommu_ops() and allow iommufd to call
> domain_alloc_paging_flags() and fail everything else.
>
> This allows explicitly creating a HWPT under an IOAS.
>
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Jacob Pan <jacob.pan@linux.microsoft.com>
>
> ---
> v4:
> - Make iommufd_noiommu_ops const
> v3:
> - Add comment to explain the design difference over the
> legacy noiommu VFIO code.
>
> fix const hwpt
> ---
> drivers/iommu/iommufd/Makefile | 1 +
> drivers/iommu/iommufd/hw_pagetable.c | 11 ++-
> drivers/iommu/iommufd/hwpt_noiommu.c | 102 ++++++++++++++++++++++++
> drivers/iommu/iommufd/iommufd_private.h | 2 +
> 4 files changed, 114 insertions(+), 2 deletions(-)
> create mode 100644 drivers/iommu/iommufd/hwpt_noiommu.c
>
> diff --git a/drivers/iommu/iommufd/Makefile b/drivers/iommu/iommufd/Makefile
> index 71d692c9a8f4..2b1a020b14a6 100644
> --- a/drivers/iommu/iommufd/Makefile
> +++ b/drivers/iommu/iommufd/Makefile
> @@ -10,6 +10,7 @@ iommufd-y := \
> vfio_compat.o \
> viommu.o
>
> +iommufd-$(CONFIG_VFIO_NOIOMMU) += hwpt_noiommu.o

is it better to have a separate kconfig for it? Also, VFIO_NOIOMMU
depends on VFIO_GROUP, this does not sound good. For one, cdev's
noiommu support should not depend on groups. For two, this means the
iommufd noiommu support is only for VFIO. It closes the gate for other
userpsace drivers. Perhaps a IOMMUFD_NOIOMMU kconfig for it.

VFIO_GROUP | VFIO_DEVICE_CDEV | IOMMUFD_NOIOMMU | VFIO_NOIOMMU allowed?
-----------+------------------+-----------------+----------------------
Y | Y | Y | Y (both paths work)
Y | Y | N | N (cdev path lacks
IOMMUFD_NOIOMMU)
Y | N | Y | Y (group path only,
cdev irrelevant)
Y | N | N | Y (group path only,
cdev irrelevant)
N | Y | Y | Y (cdev path only)
N | Y | N | N (cdev path lacks
IOMMUFD_NOIOMMU)
N | N | any | N (no VFIO interface
at all)

Then VFIO_NOIOMMU Kconfig dependency is:

depends on VFIO_GROUP || VFIO_DEVICE_CDEV
depends on !VFIO_DEVICE_CDEV || IOMMUFD_NOIOMMU

> iommufd-$(CONFIG_IOMMUFD_TEST) += selftest.o
>
> obj-$(CONFIG_IOMMUFD) += iommufd.o
> diff --git a/drivers/iommu/iommufd/hw_pagetable.c b/drivers/iommu/iommufd/hw_pagetable.c
> index fe789c2dc0c9..37316d77277d 100644
> --- a/drivers/iommu/iommufd/hw_pagetable.c
> +++ b/drivers/iommu/iommufd/hw_pagetable.c
> @@ -8,6 +8,13 @@
> #include "../iommu-priv.h"
> #include "iommufd_private.h"
>
> +static const struct iommu_ops *get_iommu_ops(struct iommufd_device *idev)
> +{
> + if (IS_ENABLED(CONFIG_VFIO_NOIOMMU) && !idev->igroup->group)
> + return &iommufd_noiommu_ops;
> + return dev_iommu_ops(idev->dev);
> +}
> +
> static void __iommufd_hwpt_destroy(struct iommufd_hw_pagetable *hwpt)
> {
> if (hwpt->domain)
> @@ -114,7 +121,7 @@ iommufd_hwpt_paging_alloc(struct iommufd_ctx *ictx, struct iommufd_ioas *ioas,
> IOMMU_HWPT_ALLOC_DIRTY_TRACKING |
> IOMMU_HWPT_FAULT_ID_VALID |
> IOMMU_HWPT_ALLOC_PASID;
> - const struct iommu_ops *ops = dev_iommu_ops(idev->dev);
> + const struct iommu_ops *ops = get_iommu_ops(idev);
> struct iommufd_hwpt_paging *hwpt_paging;
> struct iommufd_hw_pagetable *hwpt;
> int rc;
> @@ -229,7 +236,7 @@ iommufd_hwpt_nested_alloc(struct iommufd_ctx *ictx,
> struct iommufd_device *idev, u32 flags,
> const struct iommu_user_data *user_data)
> {
> - const struct iommu_ops *ops = dev_iommu_ops(idev->dev);
> + const struct iommu_ops *ops = get_iommu_ops(idev);
> struct iommufd_hwpt_nested *hwpt_nested;
> struct iommufd_hw_pagetable *hwpt;
> int rc;
> diff --git a/drivers/iommu/iommufd/hwpt_noiommu.c b/drivers/iommu/iommufd/hwpt_noiommu.c
> new file mode 100644
> index 000000000000..1c8cae02beec
> --- /dev/null
> +++ b/drivers/iommu/iommufd/hwpt_noiommu.c
> @@ -0,0 +1,102 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (c) 2021-2022, NVIDIA CORPORATION & AFFILIATES
> + */
> +#include <linux/iommu.h>
> +#include <linux/generic_pt/iommu.h>

how about moving it to the first one? :)

> +#include "iommufd_private.h"
> +
> +static const struct iommu_domain_ops noiommu_amdv1_ops;
> +
> +struct noiommu_domain {
> + union {
> + struct iommu_domain domain;
> + struct pt_iommu_amdv1 amdv1;
> + };
> + spinlock_t lock;
> +};
> +PT_IOMMU_CHECK_DOMAIN(struct noiommu_domain, amdv1.iommu, domain);
> +
> +static void noiommu_change_top(struct pt_iommu *iommu_table,
> + phys_addr_t top_paddr, unsigned int top_level)
> +{
> +}
> +
> +static spinlock_t *noiommu_get_top_lock(struct pt_iommu *iommupt)
> +{
> + struct noiommu_domain *domain =
> + container_of(iommupt, struct noiommu_domain, amdv1.iommu);
> +
> + return &domain->lock;
> +}
> +
> +static const struct pt_iommu_driver_ops noiommu_driver_ops = {
> + .get_top_lock = noiommu_get_top_lock,
> + .change_top = noiommu_change_top,
> +};
> +
> +static struct iommu_domain *
> +noiommu_alloc_paging_flags(struct device *dev, u32 flags,
> + const struct iommu_user_data *user_data)
> +{
> + struct pt_iommu_amdv1_cfg cfg = {};
> + struct noiommu_domain *dom;
> + int rc;
> +
> + if (flags || user_data)
> + return ERR_PTR(-EOPNOTSUPP);
> +
> + cfg.common.hw_max_vasz_lg2 = 64;
> + cfg.common.hw_max_oasz_lg2 = 52;
> + cfg.starting_level = 2;
> + cfg.common.features =
> + (BIT(PT_FEAT_DYNAMIC_TOP) | BIT(PT_FEAT_AMDV1_ENCRYPT_TABLES) |
> + BIT(PT_FEAT_AMDV1_FORCE_COHERENCE));
> +
> + dom = kzalloc(sizeof(*dom), GFP_KERNEL);
> + if (!dom)
> + return ERR_PTR(-ENOMEM);
> +
> + spin_lock_init(&dom->lock);
> + dom->amdv1.iommu.nid = NUMA_NO_NODE;
> + dom->amdv1.iommu.driver_ops = &noiommu_driver_ops;
> + dom->domain.ops = &noiommu_amdv1_ops;
> +
> + /* Use mock page table which is based on AMDV1 */

"mock" may mislead reader that this is test code... may just explain why
choose AMDV1 page table.

> + rc = pt_iommu_amdv1_init(&dom->amdv1, &cfg, GFP_KERNEL);
> + if (rc) {
> + kfree(dom);
> + return ERR_PTR(rc);
> + }
> +
> + return &dom->domain;
> +}
> +
> +static void noiommu_domain_free(struct iommu_domain *iommu_domain)
> +{
> + struct noiommu_domain *domain =
> + container_of(iommu_domain, struct noiommu_domain, domain);
> +
> + pt_iommu_deinit(&domain->amdv1.iommu);
> + kfree(domain);
> +}
> +
> +/*
> + * AMDV1 is used as a dummy page table for no-IOMMU mode, similar to the
> + * iommufd selftest mock page table.
> + * Unlike legacy VFIO no-IOMMU mode, where no container level APIs are
> + * supported, this allows IOAS and hwpt objects to exist without hardware
> + * IOMMU support. IOVAs are used only for IOVA-to-PA lookups not for
> + * hardware translation in DMA.

Not sure if the above comment is helpful. To me, IOAS/hwpt operation
support is the natural advantage of using cdev. What helps here is
explaining why the specific ops are defined for noiommu_amdv1_ops. If
you'd keep it, may go in the commit message. :)

Regards,
Yi Liu

> + *
> + * This is only used with iommufd and cdev-based interfaces and does not
> + * apply to legacy VFIO group-container based noiommu mode.
> + */
> +static const struct iommu_domain_ops noiommu_amdv1_ops = {
> + IOMMU_PT_DOMAIN_OPS(amdv1),
> + .free = noiommu_domain_free,
> +};
> +
> +const struct iommu_ops iommufd_noiommu_ops = {
> + .domain_alloc_paging_flags = noiommu_alloc_paging_flags,
> +};
> diff --git a/drivers/iommu/iommufd/iommufd_private.h b/drivers/iommu/iommufd/iommufd_private.h
> index 6ac1965199e9..2682b5baa6e9 100644
> --- a/drivers/iommu/iommufd/iommufd_private.h
> +++ b/drivers/iommu/iommufd/iommufd_private.h
> @@ -464,6 +464,8 @@ static inline void iommufd_hw_pagetable_put(struct iommufd_ctx *ictx,
> refcount_dec(&hwpt->obj.users);
> }
>
> +extern const struct iommu_ops iommufd_noiommu_ops;
> +
> struct iommufd_attach;
>
> struct iommufd_group {