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Re: [Qemu-devel] [RFC PATCH v3 3/3] VFIO Type1 IOMMU change: to support

From: Tian, Kevin
Subject: Re: [Qemu-devel] [RFC PATCH v3 3/3] VFIO Type1 IOMMU change: to support with iommu and without iommu
Date: Tue, 7 Jun 2016 02:47:10 +0000

> From: Dong Jia
> Sent: Monday, June 06, 2016 2:59 PM


> Channel I/O is quite different to PCI, so I copied some more details
> here. Hope these could help.
> Channel subsystem:
> The channel subsystem directs the flow of information between I/O devices
> and main storage. It relieves CPUs of the task of communicating directly
> with I/O devices and permits data processing to proceed concurrently with
> I/O processing.
> Channel path:
> The channel subsystem uses one or more channel paths as the communication
> link in managing the flow of information to or from I/O devices.
> Subchannel:
> Within the channel subsystem are subchannels. One subchannel of type I/O
> is provided for and dedicated to each I/O device accessible to the channel
>                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> subsystem.
> Control unit:
> A control unit implements a standardized interface which translates between
> the Channel Subsystem and the actual device. It does this by adapting the
> characteristics of each device so that it can respond to the standard form
> of control provided by the channel subsystem.
> Channel Path:
> The channel subsystem communicates with the I/O devices by means of
> channel paths between the channel subsystem and control units.
> +-------------------+
> | channel subsystem |
> +-------------------+
> |                   |
> |   +----------+    |              +--------------+    +------------+
> |   |subchannel|    | channel path | control unit |    | I/O device |
> |   +---------------------------------------------------------------+
> |   | subchno  |    |              |              |    |    devno   |
> |   +----------+    |              +--------------+    +------------+
> |                   |
> +-------------------+
> There is no concept of ccw-device registers by the subchannel. Control
> unit will interact with the device, collect the I/O result, and inform
> the result to the subchannel.
> So it seems to me, there is no needs to provide region info for
> isolation. As mentioned above, the isolation is quite natural.
> Please correct me in case I misunderstood some of the concepts in your
> questions and gave irrelevant answers. :>

Thanks for above background which is very useful. Several follow-up Qs:

1) Does it mean that VFIO is managing resource in subchannel level, so Qemu
can only operate subchannels assigned to itself (then emulate as the complete
channel io sub-system to guest)?

2) How are ccw commands associated with a subchannel? Are they submitted
through a dedicated subchannel interface (so VFIO can easily map that interface)
or that subchannel is specified by a special ccw cmd (means VFIO-ccw needs
to scan cmds to avoid malicious attempts on non-assigned subchannels)?


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