Re: [PATCH v2 08/16] iommu: define and export iommu_access_remote_vm()

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

Hi Fenghua,

On Mon, Mar 06, 2023 at 08:31:30AM -0800, Fenghua Yu wrote:
> Define and export iommu_access_remote_vm() to allow IOMMU related
> drivers to access user address space by PASID.
> 
> The IDXD driver would like to use it to write the user's completion
> record that the hardware device is not able to write to due to user
> page fault.
> 
> Without the API, it's complex for IDXD driver to copy completion record
> to a process' fault address for two reasons:
> 1. access_remote_vm() is not exported and shouldn't be exported for
>    drivers because drivers may easily cause mm reference issue.
> 2. user frees fault address pages to trigger fault by IDXD device.
> 
> The driver has to call iommu_sva_find(), kthread_use_mm(), re-implement
> majority of access_remote_vm() etc to access remote vm.
> 
> This IOMMU specific API hides these details and provides a clean interface
> for idxd driver and potentially other IOMMU related drivers.
> 
> Suggested-by: Alistair Popple <apopple@nvidia.com>
> Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
> Cc: Joerg Roedel <joro@8bytes.org>
> Cc: Will Deacon <will@kernel.org>
> Cc: Robin Murphy <robin.murphy@arm.com>
> Cc: Alistair Popple <apopple@nvidia.com>
> Cc: Lorenzo Stoakes <lstoakes@gmail.com>
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: iommu@lists.linux.dev
> ---
> v2:
> - Define and export iommu_access_remote_vm() for IDXD driver to write
>   completion record to user address space. This change removes
>   patch 8 and 9 in v1 (Alistair Popple)
> 
>  drivers/iommu/iommu-sva.c | 35 +++++++++++++++++++++++++++++++++++
>  include/linux/iommu.h     |  9 +++++++++
>  2 files changed, 44 insertions(+)
> 
> diff --git a/drivers/iommu/iommu-sva.c b/drivers/iommu/iommu-sva.c
> index 24bf9b2b58aa..1d7a0aee58f7 100644
> --- a/drivers/iommu/iommu-sva.c
> +++ b/drivers/iommu/iommu-sva.c
> @@ -71,6 +71,41 @@ struct mm_struct *iommu_sva_find(ioasid_t pasid)
>  }
>  EXPORT_SYMBOL_GPL(iommu_sva_find);
>  
> +/**
> + * iommu_access_remote_vm - access another process' address space by PASID
> + * @pasid:	Process Address Space ID assigned to the mm
> + * @addr:	start address to access
> + * @buf:	source or destination buffer
> + * @len:	number of bytes to transfer
> + * @gup_flags:	flags modifying lookup behaviour
> + *
> + * Another process' address space is found by PASID. A reference on @mm
> + * is taken and released inside the function.
> + *
> + * Return: number of bytes copied from source to destination.
> + */
> +int iommu_access_remote_vm(ioasid_t pasid, unsigned long addr, void *buf,
> +			   int len, unsigned int gup_flags)
> +{
> +	struct mm_struct *mm;
> +	int copied;
> +
> +	mm = iommu_sva_find(pasid);

The ability to find a mm by PASID is being removed, see 
https://lore.kernel.org/linux-iommu/20230301235646.2692846-4-jacob.jun.pan@linux.intel.com/

Thanks,
Jean

> +	if (IS_ERR_OR_NULL(mm))
> +		return 0;
> +
> +	/*
> +	 * A reference on @mm has been held by mmget_not_zero()
> +	 * during iommu_sva_find().
> +	 */
> +	copied = access_remote_vm(mm, addr, buf, len, gup_flags);
> +	/* The reference is released. */
> +	mmput(mm);
> +
> +	return copied;
> +}
> +EXPORT_SYMBOL_GPL(iommu_access_remote_vm);
> +
>  /**
>   * iommu_sva_bind_device() - Bind a process address space to a device
>   * @dev: the device
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index 6595454d4f48..414a46a53799 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -1177,6 +1177,8 @@ struct iommu_sva *iommu_sva_bind_device(struct device *dev,
>  					struct mm_struct *mm);
>  void iommu_sva_unbind_device(struct iommu_sva *handle);
>  u32 iommu_sva_get_pasid(struct iommu_sva *handle);
> +int iommu_access_remote_vm(ioasid_t pasid, unsigned long addr, void *buf,
> +			   int len, unsigned int gup_flags);
>  #else
>  static inline struct iommu_sva *
>  iommu_sva_bind_device(struct device *dev, struct mm_struct *mm)
> @@ -1192,6 +1194,13 @@ static inline u32 iommu_sva_get_pasid(struct iommu_sva *handle)
>  {
>  	return IOMMU_PASID_INVALID;
>  }
> +
> +static inline int iommu_access_remote_vm(ioasid_t pasid, unsigned long addr,
> +					 void *buf, int len,
> +					 unsigned int gup_flags)
> +{
> +	return 0;
> +}
>  #endif /* CONFIG_IOMMU_SVA */
>  
>  #endif /* __LINUX_IOMMU_H */
> -- 
> 2.37.1
> 
>