Re: [Qemu-devel] [RFC 00/15] qmp: Report supported device types on 'query-machines'

[Eduardo Habkost]

On Wed, Nov 23, 2016 at 06:43:16PM +0200, Marcel Apfelbaum wrote:
> On 11/22/2016 03:11 AM, Eduardo Habkost wrote:
> > The Problem
> > ===========
> > 
> > Currently management software has no way to find out which device
> > types can be plugged in a machine, unless the machine is already
> > initialized.
> > 
> 
> Hi Eduardo,
> Thank you for this interesting series. I think this is a problem
> worth addressing.
> 
> > Even after the machine is initialized, there's no way to map
> > existing bus types to supported device types unless management
> > software hardcodes the mapping between bus types and device
> > types.
> > 
> 
> Here I am a little lost.
> 
> We are going for machine => supported devices or
> bus-type => supported devices?

On this series, we go for machine-type => supported-devices.

A bus-type => supported-devices map wouldn't work because
different PCIe bus instances might accept different types of
devices (so supported-devices depend on the specific bus
instance, not only on the bus-type).

v2 will probably be more detailed. I plan to change it to:

query-machine(machine-type) => list of BusInfo

BusInfo would contain:
 * bus-type
 * bus-path
 * accepted-device-types (list of type/interface names)

> 
> > Example: floppy support on q35 vs i440fx
> > ----------------------------------------
> > 
> > There's no way for libvirt to find out that there's no floppy
> > controller on pc-q35-* machine-types by default.
> > 
> 
> Again "by default". So do we want to query the init state of a machine?
> What devices are there? Or what devices *can be* there?

"by default" means what's present when using "-machine <machine>"
with no extra -device arguments.

We want to know what _buses_ are always there. Which in turn lets
management know which _device_ types _can_ be plugged.

> 
> > With this series, pc-i440fx-* will report "floppy" as a supported
> > device type, but pc-q35-* will not.
> > 
> > Example: Legacy PCI vs vs PCIe devices
> > --------------------------------------
> > 
> > Some devices require a PCIe bus to be available, others work on
> > both legacy PCI and PCIe, while others work only on a legacy PCI
> > bus.
> > 
> > Currently management software has no way to know which devices
> > can be added to a given machine, unless it hardcodes machine-type
> > names and device-types names.
> > 
> 
> Again it seems a double problem, machine => devices vs pci/pcie bus => 
> devices.
> The bus => devices match is not related to a machine type.

A bus-type => device-type match would not depend on the
machine-type, but it would not be useful: different bus instances
can accept different device-types (and the way the bus topology
is configured depend on the machine-type).


> 
> > The Proposed Interface
> > ======================
> > 
> > This series adds a new field to the output of 'query-machines':
> > 'supported-device-types'. It will contain a list of QOM type
> > names, that can be used to find the list of device types that can
> > be plugged in the machine by default.
> 
> What do you mean "by default"? Without bridges or part of the machine itself?

I mean "when you just run -machine with no extra -device
arguments".

> 
>  The type names reported on
> > the new field can then be used as the 'implements' argument on
> > the 'qom-list-types' command, to find out which device types can
> > be plugged on the machine.
> > 
> > Example output
> > --------------
> > 
> >   (QEMU) query-machines
> >   {
> >     "return": [
> >         [...]
> >         {
> >             "supported-device-types": [
> >                 "sys-bus-device"
> 
> 
> I don't know how "sys-bus-device" can help us... :)

Yes, I added comments about it below. :)

> 
> >             ],
> >             "cpu-max": 1,
> >             "hotpluggable-cpus": false,
> >             "name": "none"
> >         },
> >         [...]
> >         {
> >             "supported-device-types": [
> >                 "sys-bus-device"
> >             ],
> >             "cpu-max": 1,
> >             "hotpluggable-cpus": false,
> >             "name": "xenpv"
> >         },
> >         [...]
> >         {
> >             "supported-device-types": [
> >                 "sys-bus-device",
> >                 "floppy",
> >                 "i2c-slave",
> >                 "pci-device",
> >                 "isa-device",
> >                 "ide-device"
> 
> Is don't know is this high level classification is useful,
> here is an example:
> 
>    pvi-device is supported => then we look for all pci devices?
> But what if some pci devices make sense on a machine type,
> but not on another?

If not all pci devices are supported, then the machine must not
return "pci-device" as supported. We need to define a new
type/interface name that would be implemented only by the
supported devices. e.g. "legacy-pci-device".

> 
> 
> 
> >             ],
> >             "name": "pc-i440fx-2.8",
> >             "alias": "pc",
> >             "is-default": true,
> >             "cpu-max": 255,
> >             "hotpluggable-cpus": true
> >         },
> >         [...]
> >         {
> >             "supported-device-types": [
> >                 "sys-bus-device",
> >                 "floppy",
> >                 "isa-device",
> >                 "ide-device"
> >             ],
> >             "cpu-max": 1,
> >             "hotpluggable-cpus": true,
> >             "name": "isapc"
> >         },
> >         [...]
> >         {
> >             "supported-device-types": [
> >                 "sys-bus-device",
> >                 "floppy",
> >                 "i2c-slave",
> >                 "pci-device",
> >                 "isa-device",
> >                 "ide-device"
> >             ],
> >             "cpu-max": 128,
> >             "hotpluggable-cpus": true,
> >             "name": "xenfv"
> >         },
> >         [...]
> >         {
> >             "alias": "q35",
> >             "supported-device-types": [
> >                 "sys-bus-device",
> >                 "i2c-slave",
> >                 "PCIE-device",
> >                 "isa-device",
> >                 "ide-device"
> >             ],
> >             "cpu-max": 288,
> >             "hotpluggable-cpus": true,
> >             "name": "pc-q35-2.8"
> >         },
> >         [...]
> >     ]
> >   }
> > 
> > Considered alternatives
> > =======================
> > 
> > Indirect mapping (machine => bus => device)
> > -------------------------------------------
> > 
> > This RFC implements a mechanism to implement ax
> >   machine-type => supported-device-types
> > mapping. An alternative solution I considered was to expose an
> > indirect mapping:
> >   machine-type => default-bus-types
> > followed by
> >   bus-type => supported-device-types.
> > 
> 
> As I previously stated, I don't know if it helps. bus-type
> can support different devices on different archs.

Yes. After learning a bit more about PCIe, I am convinced that a
bus->type => device-types mapping won't help.

> 
> > But exposing only the resulting supported device-types list
> > imposes less restrictions on how the device and bus type
> > hierarchy is implemented inside QEMU. There's still a
> >   machine-type => bus-type => device-type
> > mapping implemented internally, but it is an implementation
> > detail on the current version, and not part of the
> > externally-visible interface.
> > 
> 
> Good, I personally don't like this "pass".
> 
> > The Implementation
> > ==================
> > 
> > This add a new field to MachineClass: default_bus_types, and a
> > new field to BusClass: supported_device_type.
> > 
> > The two fields are used to build the list of supported device
> > types for a given machine. On most cases, the normal QOM type
> > hierarchy is used to define the set of supported devices for a
> > bus. On the case of PCIe buses, a INTERFACE_PCIE_DEVICE interface
> > name was introduced, to indicate PCIe-capable devices.
> > 
> > This means we are duplicating information in some cases:
> > 
> > * BusClass::supported_device_type duplicates knowlege that is
> >   already encoded in DeviceClass::bus_type.
> > 
> >   To make sure both fields agree with each other, a new
> >   device_class_set_bus_type() wrapper was added, to perform
> >   additional validation.
> > 
> > * MachineClass::default_bus_type duplicates knowledge that is
> >   already encoded in the machine init function.
> > 
> >   To make sure the information is correct, a qmp-machine-info.py
> >   test case is added, that will validate the
> >   supported-device-types field based on the buses created by the
> >   machine.
> > 
> > * PCIDeviceClass::is_express and INTERFACE_PCIE_DEVICE
> >   both encode the same information about a PCI device class.
> > 
> >   A future version of this series may include a
> >   class_base_post_init hook that will allow TYPE_PCI_DEVICE to
> >   validate/update is_express and the interface list to ensure
> >   both are always consistent.
> > 
> > Test Code
> > ---------
> > 
> > qdev-based test code for the new field was implemented in a
> > Python script.  Some extra support was implemented in
> > tests/Makefile.include, scripts/qemu.py and scripts/qtest.py to
> > allow the test to be implemented.
> > 
> > Limitations
> > ===========
> > 
> > TYPE_SYS_BUS_DEVICE is too generic
> > ----------------------------------
> > 
> > Currently all machines have a TYPE_SYS_BUS bus, meaning all
> > TYPE_SYS_BUS_DEVICE subclasses are reported as supported.
> > 
> 
> Agreed, this is a problem.
> 
> > The current solution in this series is to report
> > TYPE_SYS_BUS_DEVICE as supported by all machines. But we could
> > gradually add arch-specific or machine-family-specific interface
> > names that can be used on devices that are meant to work with
> > only a subset of TYPE_SYS_BUS_DEVICE subclasses.
> > 
> > A future version of this series may remove TYPE_SYS_BUS_DEVICE
> > from the supported-device-types output, and return a
> > arch-specific or machine-family-specific interface name to
> > restrict management software to a subset of TYPE_SYS_BUS_DEVICE
> > subclasses.
> > 
> > PCI vs PCIe
> > -----------
> > 
> > Machines with PCIe buses will report INTERFACE_PCIE_DEVICE on
> > supported-device-types.
> > 
> > Machines with legacy PCI buses will report TYPE_PCI_DEVICE on
> > supported-device-types.
> > 
> > The problem with the current approach is that PCIe devices are
> > TYPE_PCI_DEVICE subclasses. The allows PCI device classes to
> > indicate they are PCIe-capable, but there's no obvious way to
> > indicate that a device is PCIe-only. This needs to be addressed
> > in a future version of this series.
> > 
> > Suggestions are welcome.
> 
> 
> As we talked offline, I personally like an interface IPCIType
> with a field having 3 possible values {pci/pcie/hybrid}
> 
> To understand how hybrid works we need some rules, like
> "pci on pci bus/pcie on pcie bus"
> "pcie on a non-root pcie bus/pcie otherwise
> 
> I don't think we'll have a lot of rules, simple boolean fields
> for the interface should be enough.

What you propose makes sense, the only difference is that the
boolean fields would be just interface names that can be used as
the "implements" argument on qom-list-types.

e.g.:

* Hybrid PCI device-types would implement both "legacy-pci-device" and
  "pcie-device" interfaces.
* PCIe-only device-types would implement only the "pcie-device"
  interface.
* Legacy-PCI-only device-types would implement only the
  "legacy-pci-device" interface.

Then, the bus instances would have a 'accepted-device-types'
field:

* A legacy PCI bus would accept only "legacy-pci-device" devices.
* A PCIe-only bus would accept only "pcie-device" devices.
* A PCIe bus that accepts legacy PCI devices (the root bus?)
  would accept both "legacy-pci-device" and "pcie-device"
  devices.

Then, query-machines would return the list of bus instances that
machine init creates, containing the bus ID, bus type, and
accepted-device-types.

Does that make sense?

> 
> This still does not solve the problem that some devices makes
> sense only on a specific arch.

Right now, we can simply compile out arch-specific devices that
can never be plugged in a QEMU binary. But if we still want them
compiled in for some reason, we can categorize them on a
different type/interface name, and the corresponding machine-type
would tell management that their buses accept only the
arch-specific type/interface name.

> 
> > 
> > Incomplete bus lists on some machines
> > -------------------------------------
> > 
> > With this series, not all machines classes are changed to add the
> > full list of device types on the 'supported-device-types'. To
> > allow the code to be updated gradually, qmp-machine-info.py has a
> > STRICT_ARCHES variable, that will make the test code require a
> > complete device type list only on some architectures.
> > 
> > Out of scope: Configurable buses
> > --------------------------------
> > 
> > There's no way to map machine options like "usb=on|off" to
> > device-types or buses. I plan to propose a new interface that
> > allows machine options to be mapped to buses/device-types later.
> > 
> > Out of scope: Deciding where to plug devices
> > --------------------------------------------
> > 
> > Once management software discovers which devices can be plugged
> > to a machine, it still has to discover or define where devices
> > can/should/will be plugged. This is out of the scope of this
> > series.
> > 
> 
> That's a pitty :(
> I was hoping this series will solve this issue. But if it prepares
> the grounds for it is also a good step .

The bus ID will be in the scope of v2.

> 
> 
> 
> Thanks,
> Marcel
> 
> > Out of scope: Hotplug
> > ---------------------
> > 
> > The proposed interface is supposed to help management software
> > find which device types can be used when creating the VM. Device
> > hotplug is out of the scope of this series. However, the new
> > 'device-types' QOM property on bus objects could be used to find
> > out which device types can be plugged on the existing buses.
> > 
> > ---
> > Cc: address@hidden
> > Cc: Laine Stump <address@hidden>
> > 
> > Eduardo Habkost (15):
> >   qemu.py: Make logging optional
> >   qtest.py: Support QTEST_LOG environment variable
> >   qtest.py: make logging optional
> >   qtest.py: Make 'binary' parameter optional
> >   tests: Add rules to non-gtester qtest test cases
> >   qdev: Add device_type field to BusClass
> >   machine: Add MachineClass::default_buses field
> >   qmp: Add 'supported-device-types' field to 'query-machines'
> >   pci: Introduce INTERFACE_PCIE_DEVICE interface name
> >   pc: Initialize default bus lists
> >   s390x: Initialize default bus lists
> >   arm: Initialize default bus lists
> >   mips: Initialize default bus lists
> >   ppc: Initialize default bus lists
> >   qdev: Add device_class_set_bus_type() function
> > 
> >  hw/arm/aspeed.c                    |   2 +
> >  hw/arm/collie.c                    |   1 +
> >  hw/arm/cubieboard.c                |   1 +
> >  hw/arm/exynos4_boards.c            |   5 ++
> >  hw/arm/gumstix.c                   |   7 ++
> >  hw/arm/highbank.c                  |   4 ++
> >  hw/arm/imx25_pdk.c                 |   1 +
> >  hw/arm/kzm.c                       |   1 +
> >  hw/arm/musicpal.c                  |   1 +
> >  hw/arm/nseries.c                   |   2 +
> >  hw/arm/palm.c                      |   1 +
> >  hw/arm/realview.c                  |   1 +
> >  hw/arm/spitz.c                     |  10 +++
> >  hw/arm/stellaris.c                 |   4 ++
> >  hw/audio/intel-hda.c               |   9 ++-
> >  hw/block/fdc.c                     |  17 +++--
> >  hw/block/nvme.c                    |   4 ++
> >  hw/char/virtio-serial-bus.c        |   3 +-
> >  hw/core/bus.c                      |   9 +++
> >  hw/core/machine.c                  |  18 ++++-
> >  hw/core/qdev.c                     |   8 +++
> >  hw/core/sysbus.c                   |   3 +-
> >  hw/i2c/core.c                      |   9 ++-
> >  hw/i386/pc_piix.c                  |  13 ++++
> >  hw/i386/pc_q35.c                   |   4 ++
> >  hw/ide/qdev.c                      |   3 +-
> >  hw/input/adb.c                     |   9 ++-
> >  hw/ipack/ipack.c                   |   9 ++-
> >  hw/isa/isa-bus.c                   |   3 +-
> >  hw/mips/mips_malta.c               |   7 ++
> >  hw/mips/mips_r4k.c                 |   2 +
> >  hw/misc/auxbus.c                   |   3 +-
> >  hw/net/e1000e.c                    |   4 ++
> >  hw/net/vmxnet3.c                   |   4 ++
> >  hw/pci-bridge/ioh3420.c            |   4 ++
> >  hw/pci-bridge/xio3130_downstream.c |   4 ++
> >  hw/pci/pci.c                       |  16 ++++-
> >  hw/ppc/e500plat.c                  |   3 +
> >  hw/ppc/mac_newworld.c              |   4 ++
> >  hw/ppc/mac_oldworld.c              |   3 +
> >  hw/ppc/mpc8544ds.c                 |   4 ++
> >  hw/ppc/ppc440_bamboo.c             |   1 +
> >  hw/ppc/prep.c                      |   4 ++
> >  hw/ppc/spapr_vio.c                 |   3 +-
> >  hw/s390x/css-bridge.c              |   2 +
> >  hw/s390x/event-facility.c          |   3 +-
> >  hw/s390x/s390-pci-bus.c            |   9 ++-
> >  hw/s390x/s390-virtio-ccw.c         |   6 ++
> >  hw/s390x/virtio-ccw.c              |   2 +-
> >  hw/scsi/megasas.c                  |   7 ++
> >  hw/scsi/scsi-bus.c                 |   3 +-
> >  hw/scsi/vmw_pvscsi.c               |   1 +
> >  hw/sd/core.c                       |   7 ++
> >  hw/sd/sd.c                         |   2 +-
> >  hw/ssi/ssi.c                       |   9 ++-
> >  hw/usb/bus.c                       |   3 +-
> >  hw/usb/dev-smartcard-reader.c      |   9 ++-
> >  hw/usb/hcd-xhci.c                  |   4 ++
> >  hw/vfio/pci.c                      |   4 ++
> >  hw/virtio/virtio-bus.c             |   1 +
> >  hw/virtio/virtio-pci.c             |   4 ++
> >  hw/virtio/virtio.c                 |   2 +-
> >  include/hw/boards.h                |   5 ++
> >  include/hw/pci/pci.h               |   3 +
> >  include/hw/qdev-core.h             |   4 ++
> >  qapi-schema.json                   |   9 ++-
> >  scripts/qemu.py                    |  25 +++++--
> >  scripts/qtest.py                   |  15 +++-
> >  tests/Makefile.include             |  39 ++++++++++-
> >  tests/qmp-machine-info.py          | 138 
> > +++++++++++++++++++++++++++++++++++++
> >  vl.c                               |  11 +++
> >  71 files changed, 518 insertions(+), 37 deletions(-)
> >  create mode 100755 tests/qmp-machine-info.py
> > 
> 
> 

-- 
Eduardo