RE: [RFC] /dev/ioasid uAPI proposal

[Parav Pandit <parav@nvidia.com>]

> From: Tian, Kevin <kevin.tian@intel.com>
> Sent: Thursday, May 27, 2021 1:28 PM
> /dev/ioasid provides an unified interface for managing I/O page tables for
> devices assigned to userspace. Device passthrough frameworks (VFIO, vDPA,
> etc.) are expected to use this interface instead of creating their own logic to
> isolate untrusted device DMAs initiated by userspace.
> 
> This proposal describes the uAPI of /dev/ioasid and also sample sequences
> with VFIO as example in typical usages. The driver-facing kernel API provided
> by the iommu layer is still TBD, which can be discussed after consensus is
> made on this uAPI.
> 
> It's based on a lengthy discussion starting from here:
> 	https://lore.kernel.org/linux-
> iommu/20210330132830.GO2356281@nvidia.com/
> 
> It ends up to be a long writing due to many things to be summarized and
> non-trivial effort required to connect them into a complete proposal.
> Hope it provides a clean base to converge.

Thanks for the detailed RFC. Digesting it...

[..]
> 2.1. /dev/ioasid uAPI
> +++++++++++++++++
> /*
>   * Register user space memory where DMA is allowed.
>   *
>   * It pins user pages and does the locked memory accounting so sub-
>   * sequent IOASID_MAP/UNMAP_DMA calls get faster.
>   *
>   * When this ioctl is not used, one user page might be accounted
>   * multiple times when it is mapped by multiple IOASIDs which are
>   * not nested together.
>   *
>   * Input parameters:
>   *	- vaddr;
>   *	- size;
>   *
>   * Return: 0 on success, -errno on failure.
>   */	
It appears that this is only to make map ioctl faster apart from accounting.
It doesn't have any ioasid handle input either.

In that case, can it be a new system call? Why does it have to be under /dev/ioasid?
For example few years back such system call mpin() thought was proposed in [1].

Or a new MAP_PINNED flag is better approach to achieve in single mmap() call?

> #define IOASID_REGISTER_MEMORY	_IO(IOASID_TYPE, IOASID_BASE + 1)
> #define IOASID_UNREGISTER_MEMORY	_IO(IOASID_TYPE,
> IOASID_BASE + 2)

[1] https://lwn.net/Articles/600502/