Re: [RFC PATCH] vfio/iommu_type1: Multi-IOMMU domain support

[Alex Williamson]

On Mon, 2014-01-27 at 16:17 -0500, Don Dutile wrote:
> On 01/20/2014 11:21 AM, Alex Williamson wrote:
> > On Mon, 2014-01-20 at 14:45 +0000, Varun Sethi wrote:
> >>
> >>> -----Original Message-----
> >>> From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> >>> Sent: Saturday, January 18, 2014 2:06 AM
> >>> To: Sethi Varun-B16395
> >>> Cc: iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> >>> Subject: [RFC PATCH] vfio/iommu_type1: Multi-IOMMU domain support
> >>>
> >>> RFC: This is not complete but I want to share with Varun the dirrection
> >>> I'm thinking about.  In particular, I'm really not sure if we want to
> >>> introduce a "v2" interface version with slightly different unmap
> >>> semantics.  QEMU doesn't care about the difference, but other users
> >>> might.  Be warned, I'm not even sure if this code works at the moment.
> >>> Thanks,
> >>>
> >>> Alex
> >>>
> >>>
> >>> We currently have a problem that we cannot support advanced features of
> >>> an IOMMU domain (ex. IOMMU_CACHE), because we have no guarantee that
> >>> those features will be supported by all of the hardware units involved
> >>> with the domain over its lifetime.  For instance, the Intel VT-d
> >>> architecture does not require that all DRHDs support snoop control.  If
> >>> we create a domain based on a device behind a DRHD that does support
> >>> snoop control and enable SNP support via the IOMMU_CACHE mapping option,
> >>> we cannot then add a device behind a DRHD which does not support snoop
> >>> control or we'll get reserved bit faults from the SNP bit in the
> >>> pagetables.  To add to the complexity, we can't know the properties of a
> >>> domain until a device is attached.
> >> [Sethi Varun-B16395] Effectively, it's the same iommu and iommu_ops
> >> are common across all bus types. The hardware feature differences are
> >> abstracted by the driver.
> >
> > That's a simplifying assumption that is not made anywhere else in the
> > code.  The IOMMU API allows entirely independent IOMMU drivers to
> > register per bus_type.  There is no guarantee that all devices are
> > backed by the same IOMMU hardware unit or make use of the same
> > iommu_ops.
> >
> >>> We could pass this problem off to userspace and require that a separate
> >>> vfio container be used, but we don't know how to handle page accounting
> >>> in that case.  How do we know that a page pinned in one container is the
> >>> same page as a different container and avoid double billing the user for
> >>> the page.
> >>>
> >>> The solution is therefore to support multiple IOMMU domains per
> >>> container.  In the majority of cases, only one domain will be required
> >>> since hardware is typically consistent within a system.  However, this
> >>> provides us the ability to validate compatibility of domains and support
> >>> mixed environments where page table flags can be different between
> >>> domains.
> >>>
> >>> To do this, our DMA tracking needs to change.  We currently try to
> >>> coalesce user mappings into as few tracking entries as possible.  The
> >>> problem then becomes that we lose granularity of user mappings.  We've
> >>> never guaranteed that a user is able to unmap at a finer granularity than
> >>> the original mapping, but we must honor the granularity of the original
> >>> mapping.  This coalescing code is therefore removed, allowing only unmaps
> >>> covering complete maps.  The change in accounting is fairly small here, a
> >>> typical QEMU VM will start out with roughly a dozen entries, so it's
> >>> arguable if this coalescing was ever needed.
> >>>
> >>> We also move IOMMU domain creation to the point where a group is attached
> >>> to the container.  An interesting side-effect of this is that we now have
> >>> access to the device at the time of domain creation and can probe the
> >>> devices within the group to determine the bus_type.
> >>> This finally makes vfio_iommu_type1 completely device/bus agnostic.
> >>> In fact, each IOMMU domain can host devices on different buses managed by
> >>> different physical IOMMUs, and present a single DMA mapping interface to
> >>> the user.  When a new domain is created, mappings are replayed to bring
> >>> the IOMMU pagetables up to the state of the current container.  And of
> >>> course, DMA mapping and unmapping automatically traverse all of the
> >>> configured IOMMU domains.
> >>>
> >> [Sethi Varun-B16395] This code still checks to see that devices being
> >> attached to the domain are connected to the same bus type. If we
> >> intend to merge devices from different bus types but attached to
> >> compatible domains in to a single domain, why can't we avoid the bus
> >> check? Why can't we remove the bus dependency from domain allocation?
> >
> > So if I were to test iommu_ops instead of bus_type (ie. assume that if a
> > if an IOMMU driver manages iommu_ops across bus_types that it can accept
> > the devices), would that satisfy your concern?
> >
> > It may be possible to remove the bus_type dependency from domain
> > allocation, but the IOMMU API currently makes the assumption that
> > there's one IOMMU driver per bus_type.  Your fix to remove the bus_type
> > dependency from iommu_domain_alloc() adds an assumption that there is
> > only one IOMMU driver for all bus_types.  That may work on your
> > platform, but I don't think it's a valid assumption in the general case.
> > If you'd like to propose alternative ways to remove the bus_type
> > dependency, please do.  Thanks,

> >
> Making iommu-ops per-bus, and not per-bus-type would solve the problem as well,
> as Joerg tried to do at one point.  ... would layer the proper IOMMU for a given
> device to the bus it masters. (makes more sense if thought in context of bus's &
> devices as objects, and object-oriented semantics ... a device would ask its bus
> for it's mapping services, not a 'bus-type'.

Certainly that's where we'll need to be eventually, but it doesn't solve
anything for us right now.  We can get everything we need with the IOMMU
API now and it should be trivial to update for per-bus iommu_ops should
they happen.  Thanks,

Alex

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