Re: [PATCH 12/14] iommufd: Add APIs to preserve/unpreserve a vfio cdev

[Samiullah Khawaja]

On Wed, Mar 25, 2026 at 08:24:24PM +0000, Pranjal Shrivastava wrote:
> On Tue, Feb 03, 2026 at 10:09:46PM +0000, Samiullah Khawaja wrote:
> > Add APIs that can be used to preserve and unpreserve a vfio cdev. Use
> > the APIs exported by the IOMMU core to preserve/unpreserve device. Pass
> > the LUO preservation token of the attached iommufd into IOMMU preserve
> > device API. This establishes the ownership of the device with the
> > preserved iommufd.
> >
> > Signed-off-by: Samiullah Khawaja <skhawaja@xxxxxxxxxx>
> > ---
> >  drivers/iommu/iommufd/device.c | 69 ++++++++++++++++++++++++++++++++++
> >  include/linux/iommufd.h        | 23 ++++++++++++
> >  2 files changed, 92 insertions(+)
> >
> > diff --git a/drivers/iommu/iommufd/device.c b/drivers/iommu/iommufd/device.c
> > index 4c842368289f..30cb5218093b 100644
> > --- a/drivers/iommu/iommufd/device.c
> > +++ b/drivers/iommu/iommufd/device.c
> > @@ -2,6 +2,7 @@
> >  /* Copyright (c) 2021-2022, NVIDIA CORPORATION & AFFILIATES
> >   */
> >  #include <linux/iommu.h>
> > +#include <linux/iommu-lu.h>
> >  #include <linux/iommufd.h>
> >  #include <linux/pci-ats.h>
> >  #include <linux/slab.h>
> > @@ -1661,3 +1662,71 @@ int iommufd_get_hw_info(struct iommufd_ucmd *ucmd)
> >  	iommufd_put_object(ucmd->ictx, &idev->obj);
> >  	return rc;
> >  }
> > +
> > +#ifdef CONFIG_IOMMU_LIVEUPDATE
> > +int iommufd_device_preserve(struct liveupdate_session *s,
> > +			    struct iommufd_device *idev,
> > +			    u64 *tokenp)
> > +{
> > +	struct iommufd_group *igroup = idev->igroup;
> > +	struct iommufd_hwpt_paging *hwpt_paging;
> > +	struct iommufd_hw_pagetable *hwpt;
> > +	struct iommufd_attach *attach;
> > +	int ret;
> > +
> > +	mutex_lock(&igroup->lock);
> > +	attach = xa_load(&igroup->pasid_attach, IOMMU_NO_PASID);
>
> By explicitly looking up IOMMU_NO_PASID, we skip any PASID attachments
> the device might have. Since PASID live update is NOT supported in this
> series, should we check if the pasid_attach xarray contains anything
> other than IOMMU_NO_PASID and return -EOPNOTSUPP?
>
> Otherwise, we silently fail to preserve those domains without informing
> the VMM?

VMM should be able to preserve the NO_PASID domains even if it has PASID
attachments. This is the intended behaviour, I will document it in the
uAPI docs.
> > +	if (!attach) {
> > +		ret = -ENOENT;
> > +		goto out;
> > +	}
> > +
> > +	hwpt = attach->hwpt;
> > +	hwpt_paging = find_hwpt_paging(hwpt);
> > +	if (!hwpt_paging || !hwpt_paging->lu_preserve) {
> > +		ret = -EINVAL;
> > +		goto out;
> > +	}
> > +
> > +	ret = liveupdate_get_token_outgoing(s, idev->ictx->file, tokenp);
> > +	if (ret)
> > +		goto out;
> > +
> > +	ret = iommu_preserve_device(hwpt_paging->common.domain,
> > +				    idev->dev,
> > +				    *tokenp);
> > +out:
> > +	mutex_unlock(&igroup->lock);
> > +	return ret;
> > +}
> > +EXPORT_SYMBOL_NS_GPL(iommufd_device_preserve, "IOMMUFD");
> > +
> > +void iommufd_device_unpreserve(struct liveupdate_session *s,
> > +			       struct iommufd_device *idev,
> > +			       u64 token)
> > +{
> > +	struct iommufd_group *igroup = idev->igroup;
> > +	struct iommufd_hwpt_paging *hwpt_paging;
> > +	struct iommufd_hw_pagetable *hwpt;
> > +	struct iommufd_attach *attach;
> > +
> > +	mutex_lock(&igroup->lock);
> > +	attach = xa_load(&igroup->pasid_attach, IOMMU_NO_PASID);
> > +	if (!attach) {
> > +		WARN_ON(-ENOENT);
>
> WARN_ON takes a condition.. if we want this to be printed always, why
> not WARN_ON(1, "...") ? What's the significance of having -ENOENT as a
> condition?

Will update this.
> > +		goto out;
> > +	}
> > +
> > +	hwpt = attach->hwpt;
> > +	hwpt_paging = find_hwpt_paging(hwpt);
> > +	if (!hwpt_paging || !hwpt_paging->lu_preserve) {
> > +		WARN_ON(-EINVAL);
>
> Same here for -EINVAL?

Same here.
> > +		goto out;
> > +	}
> > +
> > +	iommu_unpreserve_device(hwpt_paging->common.domain, idev->dev);
> > +out:
> > +	mutex_unlock(&igroup->lock);
> > +}
> > +EXPORT_SYMBOL_NS_GPL(iommufd_device_unpreserve, "IOMMUFD");
> > +#endif
>
> [ ----- >8 ----- ]
>
> Thanks,
> Praan

Thanks,
Sami