linux-kernel - RE: [RFC 11/20] iommu/iommufd: Add IOMMU_IOASID

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Date:   Wed, 29 Sep 2021 10:47:04 +0000
From:   "Liu, Yi L" <yi.l.liu@...el.com>
To:     Jean-Philippe Brucker <jean-philippe@...aro.org>
CC:     "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
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Subject: RE: [RFC 11/20] iommu/iommufd: Add IOMMU_IOASID_ALLOC/FREE

> From: Jean-Philippe Brucker <jean-philippe@...aro.org>
> Sent: Wednesday, September 22, 2021 9:45 PM
> 
> 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?

yes, as this open described, it may need to be a range. But not sure
if PPC requires multiple ranges or just one range. Perhaps, David may
guide there.

Regards,
Yi Liu
 
> 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!