Re: [RFC 11/20] iommu/iommufd: Add IOMMU_IOASID_ALLOC/FREE

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

On Sun, Sep 19, 2021 at 02:38:39PM +0800, Liu Yi L wrote:
> This patch adds IOASID allocation/free interface per iommufd. When
> allocating an IOASID, userspace is expected to specify the type and
> format information for the target I/O page table.
> 
> This RFC supports only one type (IOMMU_IOASID_TYPE_KERNEL_TYPE1V2),
> implying a kernel-managed I/O page table with vfio type1v2 mapping
> semantics. For this type the user should specify the addr_width of
> the I/O address space and whether the I/O page table is created in
> an iommu enfore_snoop format. enforce_snoop must be true at this point,
> as the false setting requires additional contract with KVM on handling
> WBINVD emulation, which can be added later.
> 
> Userspace is expected to call IOMMU_CHECK_EXTENSION (see next patch)
> for what formats can be specified when allocating an IOASID.
> 
> Open:
> - Devices on PPC platform currently use a different iommu driver in vfio.
>   Per previous discussion they can also use vfio type1v2 as long as there
>   is a way to claim a specific iova range from a system-wide address space.

Is this the reason for passing addr_width to IOASID_ALLOC?  I didn't get
what it's used for or why it's mandatory. But for PPC it sounds like it
should be an address range instead of an upper limit?

Thanks,
Jean

>   This requirement doesn't sound PPC specific, as addr_width for pci devices
>   can be also represented by a range [0, 2^addr_width-1]. This RFC hasn't
>   adopted this design yet. We hope to have formal alignment in v1 discussion
>   and then decide how to incorporate it in v2.
> 
> - Currently ioasid term has already been used in the kernel (drivers/iommu/
>   ioasid.c) to represent the hardware I/O address space ID in the wire. It
>   covers both PCI PASID (Process Address Space ID) and ARM SSID (Sub-Stream
>   ID). We need find a way to resolve the naming conflict between the hardware
>   ID and software handle. One option is to rename the existing ioasid to be
>   pasid or ssid, given their full names still sound generic. Appreciate more
>   thoughts on this open!