Re: [PATCH v2 02/21] docs: collect the disparate device emulation docs into one section

["Alex Bennée" <alex.bennee@linaro.org>]

Markus Armbruster <armbru@redhat.com> writes:

> Cc: QOM maintainers for additional eyes.
>
> Alex Bennée <alex.bennee@linaro.org> writes:
>
>> While we are at it add a brief preamble that explains some of the
>> common concepts in QEMU's device emulation which will hopefully lead
>> to less confusing about our dizzying command line options.
>>
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>> Message-Id: <20210714093638.21077-3-alex.bennee@linaro.org>
>> ---
>>  docs/system/device-emulation.rst          | 78 +++++++++++++++++++++++
>>  docs/system/{ => devices}/ivshmem.rst     |  0
>>  docs/system/{ => devices}/net.rst         |  0
>>  docs/system/{ => devices}/nvme.rst        |  0
>>  docs/system/{ => devices}/usb.rst         |  0
>>  docs/system/{ => devices}/virtio-pmem.rst |  0
>>  docs/system/index.rst                     |  6 +-
>>  7 files changed, 79 insertions(+), 5 deletions(-)
>>  create mode 100644 docs/system/device-emulation.rst
>>  rename docs/system/{ => devices}/ivshmem.rst (100%)
>>  rename docs/system/{ => devices}/net.rst (100%)
>>  rename docs/system/{ => devices}/nvme.rst (100%)
>>  rename docs/system/{ => devices}/usb.rst (100%)
>>  rename docs/system/{ => devices}/virtio-pmem.rst (100%)
>>
>> diff --git a/docs/system/device-emulation.rst b/docs/system/device-emulation.rst
>> new file mode 100644
>> index 0000000000..3156eeac2d
>> --- /dev/null
>> +++ b/docs/system/device-emulation.rst
>> @@ -0,0 +1,78 @@
>> +.. _device-emulation:
>> +
>> +Device Emulation
>> +----------------
>> +
>> +QEMU supports the emulation of a large number of devices from
>> +peripherals such network cards and USB devices to integrated systems
>> +on a chip (SoCs). Configuration of these is often a source of
>> +confusion so it helps to have an understanding of some of the terms
>> +used to describes devices within QEMU.
>> +
>> +Common Terms
>> +~~~~~~~~~~~~
>> +
>> +Device Front End
>> +================
>> +
>> +A device front end is how a device is presented to the guest. The type
>> +of device presented should match the hardware that the guest operating
>> +system is expecting to see. All devices can be specified with the
>> +``--device`` command line option. Running QEMU with the command line
>> +options ``--device help`` will list all devices it is aware of. Using
>> +the command line ``--device foo,help`` will list the additional
>> +configuration options available for that device.
>> +
>> +A front end is often paired with a back end, which describes how the
>> +host's resources are used in the emulation.
>> +
>> +Device Buses
>> +============
>> +
>> +All devices exist on a BUS. Depending on the machine model you choose
>
> This isn't true anymore; there are bus-less devices.  To show the
> user-pluggable ones, try
>
>     $ qemu-system-FOO -device help | grep -v '", bus'


Are they user-pluggable though? Aside from strange cases like loaders
most of them look like SoC specific adornments which I suspect would
make no sense to attach to another machine type. x86_64 seems to be a
special case has all the various CPU types show up as non-bus devices. 

>
>> +(``-M foo``) a number of buses will have been automatically created.
>> +In most cases the BUS a device is attached to can be inferred, for
>> +example PCI devices are generally automatically allocated to the next
>> +free slot of the PCI bus. However in complicated configurations you
>
> "The PCI bus" tacitly assumes there's just one.
>
> We actually pick the first bus (in qtree pre-order) that can take
> another device.  Best not to rely on the search order; if you care which
> bus to plug into, specify it with bus=ID.
>
> "Next free slot" is about the device address on the bus.  Should we
> explain the concept "device address on a bus"?
>
>> +can explicitly specify what bus a device is attached to and its
>> +address. Some devices, for example a PCI SCSI host controller, will
>> +add an additional bus to the system that other devices can be attached
>
> A device can add more than one bus.

So how about:

  Most devices will exist on a BUS of some sort. Depending on the
  machine model you choose (``-M foo``) a number of buses will have been
  automatically created. In most cases the BUS a device is attached to
  can be inferred, for example PCI devices are generally automatically
  allocated to the next free address of first PCI bus found. However in
  complicated configurations you can explicitly specify what bus
  (``bus=ID``) a device is attached to along with its address
  (``addr=N``). Some devices, for example a PCI SCSI host controller,
  will add an additional buses to the system that other devices can be
  attached to. A hypothetical chain of devices might look like:

    --device foo,bus=pci.0,addr=0,id=foo.0
    --device bar,bus=foo.0,addr=1,id=baz

  which would be a bar device (with the ID of baz) which is attached to
  the foo bus foo.0 which itself is attached to the first slot of a PCI
  bus pci.0

Maybe we should add a section about device IDs?

-- 
Alex Bennée