Re: [PATCH v1 1/8] iommu: Add iommu_group_replace_domain()

From: Jason Gunthorpe <jgg@nvidia.com>
To: Lu Baolu <baolu.lu@linux.intel.com>
Cc: Joerg Roedel <joro@8bytes.org>,
	Alex Williamson <alex.williamson@redhat.com>,
	Robin Murphy <robin.murphy@arm.com>,
	Christoph Hellwig <hch@infradead.org>,
	Kevin Tian <kevin.tian@intel.com>,
	Ashok Raj <ashok.raj@intel.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Will Deacon <will@kernel.org>,
	Dan Williams <dan.j.williams@intel.com>,
	rafael@kernel.org, Diana Craciun <diana.craciun@oss.nxp.com>,
	Cornelia Huck <cohuck@redhat.com>,
	Eric Auger <eric.auger@redhat.com>, Liu Yi L <yi.l.liu@intel.com>,
	Jacob jun Pan <jacob.jun.pan@intel.com>,
	Chaitanya Kulkarni <kch@nvidia.com>,
	Stuart Yoder <stuyoder@gmail.com>,
	Laurentiu Tudor <laurentiu.tudor@nxp.com>,
	Thierry Reding <thierry.reding@gmail.com>,
	David Airlie <airlied@linux.ie>, Daniel Vetter <daniel@ffwll.ch>,
	Jonathan Hunter <jonathanh@nvidia.com>,
	Li Yang <leoyang.li@nxp.com>, Dmitry Osipenko <digetx@gmail.com>,
	iommu@lists.linux-foundation.org, linux-pci@vger.kernel.org,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1 1/8] iommu: Add iommu_group_replace_domain()
Date: Thu, 6 Jan 2022 13:06:08 -0400	[thread overview]
Message-ID: <20220106170608.GI2328285@nvidia.com> (raw)
In-Reply-To: <20220106022053.2406748-2-baolu.lu@linux.intel.com>

On Thu, Jan 06, 2022 at 10:20:46AM +0800, Lu Baolu wrote:
> Expose an interface to replace the domain of an iommu group for frameworks
> like vfio which claims the ownership of the whole iommu group.
> 
> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
>  include/linux/iommu.h | 10 ++++++++++
>  drivers/iommu/iommu.c | 37 +++++++++++++++++++++++++++++++++++++
>  2 files changed, 47 insertions(+)
> 
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index 408a6d2b3034..66ebce3d1e11 100644
> +++ b/include/linux/iommu.h
> @@ -677,6 +677,9 @@ void iommu_device_unuse_dma_api(struct device *dev);
>  int iommu_group_set_dma_owner(struct iommu_group *group, void *owner);
>  void iommu_group_release_dma_owner(struct iommu_group *group);
>  bool iommu_group_dma_owner_claimed(struct iommu_group *group);
> +int iommu_group_replace_domain(struct iommu_group *group,
> +			       struct iommu_domain *old,
> +			       struct iommu_domain *new);
>  
>  #else /* CONFIG_IOMMU_API */
>  
> @@ -1090,6 +1093,13 @@ static inline bool iommu_group_dma_owner_claimed(struct iommu_group *group)
>  {
>  	return false;
>  }
> +
> +static inline int
> +iommu_group_replace_domain(struct iommu_group *group, struct iommu_domain *old,
> +			   struct iommu_domain *new)
> +{
> +	return -ENODEV;
> +}
>  #endif /* CONFIG_IOMMU_API */
>  
>  /**
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 72a95dea688e..ab8ab95969f5 100644
> +++ b/drivers/iommu/iommu.c
> @@ -3431,3 +3431,40 @@ bool iommu_group_dma_owner_claimed(struct iommu_group *group)
>  	return user;
>  }
>  EXPORT_SYMBOL_GPL(iommu_group_dma_owner_claimed);
> +
> +/**
> + * iommu_group_replace_domain() - Replace group's domain
> + * @group: The group.
> + * @old: The previous attached domain. NULL for none.
> + * @new: The new domain about to be attached.
> + *
> + * This is to support backward compatibility for vfio which manages the dma
> + * ownership in iommu_group level.

This should mention it can only be used with iommu_group_set_dma_owner()

> +	if (old)
> +		__iommu_detach_group(old, group);
> +
> +	if (new) {
> +		ret = __iommu_attach_group(new, group);
> +		if (ret && old)
> +			__iommu_attach_group(old, group);
> +	}

The sketchy error unwind here gives me some pause for sure. Maybe we
should define that on error this leaves the domain as NULL

Complicates vfio a tiny bit to cope with this failure but seems
cleaner than leaving it indeterminate.

Jason