Re: [Qemu-devel] Re: [RFC PATCH 4/7] ide: IOMMU support

Paul Brook
2010-07-14T20:13:47Z

> On Wed, Jul 14, 2010 at 02:53:03PM +0100, Paul Brook wrote:
> > > Memory accesses must go through the IOMMU layer.
> >
> > No. Devices should not know or care whether an IOMMU is present.
>
> There are real devices that care very much about an IOMMU. Basically all
> devices supporting ATS care about that. So I don't see a problem if the
> device emulation code of qemu also cares about present IOMMUs.
>
> > You should be adding a DeviceState argument to
> > cpu_physical_memory_{rw,map}. This should then handle IOMMU translation
> > transparently.
>
> That's not a good idea imho. With an IOMMU the device no longer accesses
> cpu physical memory. It accesses device virtual memory. Using
> cpu_physical_memory* functions in device code becomes misleading when
> the device virtual address space differs from cpu physical.

Well, ok, the function name needs fixing too. However I think the only thing
missing from the current API is that it does not provide a way to determine
which device is performing the access.

Depending how the we decide to handle IOMMU invalidation, it may also be
necessary to augment the memory_map API to allow the system to request a
mapping be revoked. However this issue is not specific to the IOMMU
implementation. Such bugs are already present on any system that allows
dynamic reconfiguration of the address space, e.g. by changing PCI BARs.

> So different
> functions for devices make a lot of sense here. Another reason for
> seperate functions is that we can extend them later to support emulation
> of ATS devices.

I disagree. ATS should be an independent feature, and is inherently bus
specific. As usual the PCI spec is not publicly available, but based on the
AMD IOMMU docs I'd say that ATS is completely independent of memory accesses -
the convention being that you trust an ATS capable device to DTRT, and
configure the bus IOMMU to apply a flat mapping for accesses from such
devices.

> > You also need to accomodate the the case where multiple IOMMU are
> > present.
>
> This, indeed, is something transparent to the device. This should be
> handled inside the iommu emulation code.

I think you've got the abstraction boundaries all wrong.

A device performs a memory access on its local bus. It has no knowledge of how
that access is routed to its destination. The device should not be aware of
any IOMMUs, in the same way that it doesn't know whether it happens to be
accessing RAM or memory mapped peripherals on another device.

Each IOMMU is fundamentally part of a bus bridge. For example the bridge
between a PCI bus and the system bus. It provides a address mapping from one
bus to another.

There should be no direct interaction between an IOMMU and a device (ignoring
ATS, which is effectively a separate data channel). Everything should be done
via the cpu_phsycial_memory_* code. Likewise on a system with multiple nested
IOMMUs there should be no direct interatcion between these.
cpu_physical_memory_* should walk the device/bus tree to determine where the
access terminates, applying mappings appropriately.

Paul