Re: [PATCH V2 01/11] iommufd: Support a HWPT without an iommu driver for noiommu

[Mostafa Saleh <smostafa@google.com>]

On Thu, Mar 12, 2026 at 08:56:27AM -0700, 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>
> ---
>  drivers/iommu/iommufd/Makefile          |  1 +
>  drivers/iommu/iommufd/hw_pagetable.c    | 11 ++-
>  drivers/iommu/iommufd/hwpt_noiommu.c    | 91 +++++++++++++++++++++++++
>  drivers/iommu/iommufd/iommufd_private.h |  2 +
>  4 files changed, 103 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-$(CONFIG_VFIO_NOIOMMU) += hwpt_noiommu.o
>  iommufd-$(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..0aa99f581ca3
> --- /dev/null
> +++ b/drivers/iommu/iommufd/hwpt_noiommu.c
> @@ -0,0 +1,91 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (c) 2021-2022, NVIDIA CORPORATION & AFFILIATES
> + */
> +#include <linux/iommu.h>
> +#include <linux/generic_pt/iommu.h>
> +#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 */
> +	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);
> +}
> +
> +static const struct iommu_domain_ops noiommu_amdv1_ops = {
> +	IOMMU_PT_DOMAIN_OPS(amdv1),

I see the appeal of re-using an existing page table implementation to
keep track of iovas which -as far as I understand- are used as tokens
for DMA pinned pages later, but maybe at least add some paragraph about
that, as it is not immediately clear and that's a different design
from the legacy noiommu VFIO code.

Thanks,
Mostafa

> +	.free = noiommu_domain_free,
> +};
> +
> +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..9c18c5eb1899 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 struct iommu_ops iommufd_noiommu_ops;
> +
>  struct iommufd_attach;
>  
>  struct iommufd_group {
> -- 
> 2.34.1
>