Re: [Qemu-devel] [PATCH v4 1/2] virtio: introduce `query-virtio' QMP command

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Markus Armbruster (armbru@redhat.com) wrote:
> Jan Dakinevich <jan.dakinevich@virtuozzo.com> writes:
> 
> > On 10/03/2017 05:02 PM, Eric Blake wrote:
> >> On 10/03/2017 07:47 AM, Jan Dakinevich wrote:
> >>> The command is intended for gathering virtio information such as status,
> >>> feature bits, negotiation status. It is convenient and useful for debug
> >>> purpose.
> >>>
> >>> The commands returns generic virtio information for virtio such as
> >>> common feature names and status bits names and information for all
> >>> attached to current machine devices.
> >>>
> >>> To retrieve names of device-specific features `get_feature_name'
> >>> callback in VirtioDeviceClass also was introduced.
> >>>
> >>> Cc: Denis V. Lunev <den@virtuozzo.com>
> >>> Signed-off-by: Jan Dakinevich <jan.dakinevich@virtuozzo.com>
> >>> ---
> >>>  hw/block/virtio-blk.c       |  21 +++++++++
> >>>  hw/char/virtio-serial-bus.c |  15 +++++++
> >>>  hw/display/virtio-gpu.c     |  13 ++++++
> >>>  hw/net/virtio-net.c         |  35 +++++++++++++++
> >>>  hw/scsi/virtio-scsi.c       |  16 +++++++
> >>>  hw/virtio/Makefile.objs     |   2 +
> >>>  hw/virtio/virtio-balloon.c  |  15 +++++++
> >>>  hw/virtio/virtio-stub.c     |   9 ++++
> >>>  hw/virtio/virtio.c          | 101 ++++++++++++++++++++++++++++++++++++++++++++
> >>>  include/hw/virtio/virtio.h  |   2 +
> >>>  qapi-schema.json            |   1 +
> >>>  qapi/virtio.json            |  94 +++++++++++++++++++++++++++++++++++++++++
> >>>  12 files changed, 324 insertions(+)
> >>>  create mode 100644 hw/virtio/virtio-stub.c
> >>>  create mode 100644 qapi/virtio.json
> >> 
> >> This creates a new .json file, but does not touch MAINTAINERS.  Our idea
> >> in splitting the .json files was to make it easier for each sub-file
> >> that needs a specific maintainer in addition to the overall *.json line
> >> for QAPI maintainers, so this may deserve a MAINTAINERS entry.
> >> 
> >
> > Ok.
> >
> >>> +++ b/qapi/virtio.json
> >>> @@ -0,0 +1,94 @@
> >>> +# -*- Mode: Python -*-
> >>> +#
> >>> +
> >>> +##
> >>> +# = Virtio devices
> >>> +##
> >>> +
> >>> +{ 'include': 'common.json' }
> >>> +
> >>> +##
> >>> +# @VirtioInfoBit:
> >>> +#
> >>> +# Named virtio bit
> >>> +#
> >>> +# @bit: bit number
> >>> +#
> >>> +# @name: bit name
> >>> +#
> >>> +# Since: 2.11.0
> >>> +#
> >>> +##
> >>> +{
> >>> +    'struct': 'VirtioInfoBit',
> >>> +    'data': {
> >>> +        'bit': 'uint64',
> >> 
> >> Why is this a 64-bit value? Are the values 0-63, or are they 1, 2, 4, 8,
> >> ...?  The documentation on 'bit number' is rather sparse.
> >
> > I would prefer `uint' here, but I don't see generic unsigned type (may
> > be, I am mistaken). I could use uint8 here, though.
> >
> >> 
> >>> +        'name': 'str'
> >> 
> >> Wouldn't an enum type be better than an open-ended string?
> >> 
> >
> > Bit names are not known here, they are obtained from virtio device
> > implementations.
> 
> What exactly uses these bits?
> 
> Why do these uses justify pass-through?  By pass-through, I mean the
> messenger (QEMU) merely passes them along, without understanding them.
> Defeats introspection.

It should be noted originally it was HMP - this was just a debug command
and it's only getting the need to be introspectable because people
insisted it had a QMP version.

I think the intent is to print all flags, even ones we dont yet
understand.

Dave

> >>> +    }
> >>> +}
> >>> +
> >>> +##
> >>> +# @VirtioInfoDevice:
> >>> +#
> >>> +# Information about specific virtio device
> >>> +#
> >>> +# @qom_path: QOM path of the device
> >> 
> >> Please make this 'qom-path' - new interfaces should prefer '-' over '_'.
> >
> > Ok.
> >
> >>> +#
> >>> +# @feature-names: names of device-specific features
> >>> +#
> >>> +# @host-features: bitmask of features, provided by devices
> >>> +#
> >>> +# @guest-features: bitmask of features, acknowledged by guest
> >>> +#
> >>> +# @status: virtio device status bitmask
> >>> +#
> >>> +# Since: 2.11.0
> >>> +#
> >>> +##
> >>> +{
> >>> +    'struct': 'VirtioInfoDevice',
> >>> +    'data': {
> >>> +        'qom_path': 'str',
> >>> +        'feature-names': ['VirtioInfoBit'],
> >>> +        'host-features': 'uint64',
> >>> +        'guest-features': 'uint64',
> >>> +        'status': 'uint64'
> >> 
> >> I'm wondering if this is the best representation (where the caller has
> >> to parse the integer and then lookup in feature-names what each bit of
> >> the integer represents).  But I'm not sure I have anything better off
> >> the top of my head.
> >> 
> >
> > Consider it as way to tell caller about names of supported features.
> 
> "Unsigned integer interpreted as combination of well-known bit-valued
> symbols" is a fine C interface, but a pretty horrid QMP interface.
> What's wrong with doing a set the straightforward way as "array of
> enum"?
> 
> >>> +    }
> >>> +}
> >>> +
> >>> +##
> >>> +# @VirtioInfo:
> >>> +#
> >>> +# Information about virtio devices
> >>> +#
> >>> +# @feature-names: names of common virtio features
> >>> +#
> >>> +# @status-names: names of bits which represents virtio device status
> >>> +#
> >>> +# @devices: list of per-device virtio information
> >>> +#
> >>> +# Since: 2.11.0
> >>> +#
> >>> +##
> >>> +{
> >>> +    'struct': 'VirtioInfo',
> >>> +    'data': {
> >>> +        'feature-names': ['VirtioInfoBit'],
> >> 
> >> Why is feature-names listed at two different nestings of the return value?
> >> 
> >
> > These are different feature names. First names are common and predefined
> > for all devices. Second names are device-specific.
> >
> >>> +        'status-names': ['VirtioInfoBit'],
> >>> +        'devices': ['VirtioInfoDevice']
> >>> +    }
> >>> +}
> >>> +
> >>> +
> >>> +##
> >>> +# @query-virtio:
> >>> +#
> >>> +# Returns generic and per-device virtio information
> >>> +#
> >>> +# Since: 2.11.0
> >>> +#
> >>> +##
> >>> +{
> >>> +    'command': 'query-virtio',
> >>> +    'returns': 'VirtioInfo'
> >>> +}
> >>>
> >> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK