Re: [RFC 0/5] hmp, qmp: Add some commands to introspect virtio devices

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

* Marc-André Lureau (marcandre.lureau@gmail.com) wrote:
> Hi
> 
> On Thu, Apr 2, 2020 at 12:03 PM Laurent Vivier <lvivier@redhat.com> wrote:
> >
> > This series introduces new QMP/HMP commands to dump the status
> > of a a virtio device at different levels.
> 
> Looks nice, but my feeling is that we are growing qemu with debugging
> facilities in general with HMP/QMP.
> 
> If this is only for debugging purposes, why don't we compile it only
> when --enable-debug?
> 
> A guest that would need to be debugged could have its state loaded in
> a debug version of qemu.
> 
> Alternatively, I think most of the data you provide here could
> probably be introspected via gdb scripts.
> 
> Just some thoughts

I kind of agree, but virtio is bread-and-butter, so having it available
all the time isn't a bad idea; however having said that hte same
argument would be for why having gdb scripts to examine your virtio
queues would be useful if you're dealing with a core.

Dave

> 
> >
> > 1. Main command
> >
> >  HMP Only:
> >
> >      virtio [subcommand]
> >
> >    Example:
> >
> >     List all sub-commands:
> >
> >     (qemu) virtio
> >     virtio query  -- List all available virtio devices
> >     virtio status path -- Display status of a given virtio device
> >     virtio queue-status path queue -- Display status of a given virtio queue
> >     virtio queue-element path queue [index] -- Display element of a given virtio queue
> >
> > 2. List available virtio devices in the machine
> >
> >   HMP Form:
> >
> >     virtio query
> >
> >   Example:
> >
> >     (qemu) virtio query
> >     /machine/peripheral-anon/device[3]/virtio-backend [virtio-net]
> >     /machine/peripheral-anon/device[1]/virtio-backend [virtio-serial]
> >     /machine/peripheral-anon/device[0]/virtio-backend [virtio-blk]
> >
> >   QMP Form:
> >
> >     { 'command': 'query-virtio', 'returns': ['VirtioInfo'] }
> >
> >   Example:
> >
> >   -> { "execute": "query-virtio" }
> >   <- { "return": [
> >          {
> >              "path": "/machine/peripheral-anon/device[3]/virtio-backend",
> >              "type": "virtio-net"
> >          },
> >          {
> >              "path": "/machine/peripheral-anon/device[1]/virtio-backend",
> >              "type": "virtio-serial"
> >          },
> >          {
> >              "path": "/machine/peripheral-anon/device[0]/virtio-backend",
> >              "type": "virtio-blk"
> >          }
> >        ]
> >      }
> >
> > 3. Display status of a given virtio device
> >
> >   HMP Form:
> >
> >     virtio status <path>
> >
> >   Example:
> >
> >     (qemu) virtio status /machine/peripheral-anon/device[3]/virtio-backend
> >     /machine/peripheral-anon/device[3]/virtio-backend:
> >       Device Id:        1
> >       Guest features:   0x0000000130afffa7
> >       Host features:    0x0000000179bfffe7
> >       Backend features: 0x0000000000000000
> >       Endianness:       little
> >       VirtQueues:       3
> >
> >   QMP Form:
> >
> >     { 'command': 'virtio-status',
> >       'data': { 'path': 'str' },
> >       'returns': 'VirtioStatus'
> >     }
> >
> >   Example:
> >
> >   -> { "execute": "virtio-status",
> >        "arguments": {
> >            "path": "/machine/peripheral-anon/device[3]/virtio-backend"
> >        }
> >     }
> >   <- { "return": {
> >            "backend_features": 0,
> >            "guest_features": 5111807911,
> >            "num_vqs": 3,
> >            "host_features": 6337593319,
> >            "device_endian": "little",
> >            "device_id": 1
> >        }
> >      }
> >
> > 4. Display status of a given virtio queue
> >
> >   HMP Form:
> >
> >     virtio queue-status <path> <queue>
> >
> >   Example:
> >
> >     (qemu) virtio queue-status /machine/peripheral-anon/device[3]/virtio-backend 0
> >     /machine/peripheral-anon/device[3]/virtio-backend:
> >       index:                0
> >       inuse:                0
> >       last_avail_idx:       61
> >       shadow_avail_idx:     292
> >       signalled_used:       61
> >       signalled_used_valid: 1
> >       VRing:
> >         num:         256
> >         num_default: 256
> >         align:       4096
> >         desc:        0x000000006c352000
> >         avail:       0x000000006c353000
> >         used:        0x000000006c353240
> >
> >   QMP Form:
> >
> >     { 'command': 'virtio-queue-status',
> >       'data': { 'path': 'str', 'queue': 'uint16' },
> >       'returns': 'VirtQueueStatus'
> >     }
> >
> >   Example:
> >
> >   -> { "execute": "virtio-queue-status",
> >        "arguments": {
> >            "path": "/machine/peripheral-anon/device[3]/virtio-backend",
> >            "queue": 0
> >        }
> >     }
> >   <- { "return": {
> >        "signalled_used": 373,
> >        "inuse": 0,
> >        "vring_desc": 864411648,
> >        "vring_num_default": 256,
> >        "signalled_used_valid": 1,
> >        "vring_avail": 864415744,
> >        "last_avail_idx": 373,
> >        "queue_index": 0,
> >        "vring_used": 864416320,
> >        "shadow_avail_idx": 619,
> >        "used_idx": 373,
> >        "vring_num": 256,
> >        "vring_align": 4096
> >        }
> >      }
> >
> > 5. Display element of a given virtio queue
> >
> >   HMP Form:
> >
> >     virtio queue-element <path> <queue> [index]
> >
> >   Example:
> >
> >     Dump the information of the head element of the first queue of
> >     the first virtio device::
> >
> >       (qemu) virtio queue-element/machine/peripheral-anon/device[3]/virtio-backend 0
> >       index:  67
> >       ndescs: 1
> >       descs:  addr 0x6fe69800 len 1536 (write-only)
> >
> >       (qemu) xp/128bx 0x6fe69800
> >       000000006fe69800: 0x02 0x00 0x00 0x00 0x00 0x00 0x00 0x00
> >       000000006fe69808: 0x00 0x00 0x01 0x00 0x52 0x54 0x00 0x12
> >       000000006fe69810: 0x34 0x56 0x52 0x54 0x00 0x09 0x51 0xde
> >       000000006fe69818: 0x08 0x00 0x45 0x00 0x00 0x4c 0x8f 0x32
> >
> >     device[3] is a virtio-net device and we can see in the element buffer the
> >     MAC address of the card::
> >
> >       [root@localhost ~]# ip link show ens4
> >       2: ens4: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP m0
> >           link/ether 52:54:00:12:34:56 brd ff:ff:ff:ff:ff:ff
> >
> >     and the MAC address of the gateway::
> >
> >       [root@localhost ~]# arp -a
> >       _gateway (192.168.122.1) at 52:54:00:09:51:de [ether] on ens4
> >
> >   QMP Form:
> >
> >     { 'command': 'virtio-queue-element',
> >       'data': { 'path': 'str', 'queue': 'uint16', '*index': 'uint16' },
> >       'returns': 'VirtioQueueElement'
> >     }
> >
> >   Example:
> >
> >   -> { "execute": "virtio-queue-element",
> >        "arguments": {
> >            "path": "/machine/peripheral-anon/device[3]/virtio-backend",
> >            "queue": 0
> >        }
> >     }
> >   -> { "return": {
> >           "index": 109,
> >           "len": 0,
> >           "ndescs": 1,
> >           "descs": [
> >               { "flags": 2, "len": 2048, "addr": 853145600 }
> >           ]
> >        }
> >     }
> >
> > Laurent Vivier (5):
> >   qmp: add QMP command query-virtio
> >   qmp: add QMP command virtio-status
> >   qmp: add QMP command virtio-queue-status
> >   qmp: add QMP command virtio-queue-element
> >   hmp: add virtio commands
> >
> >  Makefile                   |   2 +-
> >  Makefile.target            |   7 +-
> >  docs/system/monitor.rst    |   2 +
> >  hmp-commands-virtio.hx     | 148 ++++++++++++++++++
> >  hmp-commands.hx            |  10 ++
> >  hw/virtio/Makefile.objs    |   2 +
> >  hw/virtio/virtio-stub.c    |  32 ++++
> >  hw/virtio/virtio.c         | 313 +++++++++++++++++++++++++++++++++++++
> >  include/hw/virtio/virtio.h |   1 +
> >  include/monitor/hmp.h      |   4 +
> >  monitor/misc.c             |  17 ++
> >  qapi/Makefile.objs         |   2 +-
> >  qapi/qapi-schema.json      |   1 +
> >  qapi/virtio.json           | 302 +++++++++++++++++++++++++++++++++++
> >  tests/qtest/qmp-cmd-test.c |   1 +
> >  15 files changed, 840 insertions(+), 4 deletions(-)
> >  create mode 100644 hmp-commands-virtio.hx
> >  create mode 100644 hw/virtio/virtio-stub.c
> >  create mode 100644 qapi/virtio.json
> >
> > --
> > 2.25.1
> >
> >
> 
> 
> -- 
> Marc-André Lureau
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK